Showing papers on "Potential energy surface published in 2006"
TL;DR: A new approach for calculating reaction coordinates in complex systems based on transition path sampling and likelihood maximization is presented, which requires fewer trajectories than a single iteration of existing procedures and applies to both low and high friction dynamics.
Abstract: We present a new approach for calculating reaction coordinates in complex systems. The new method is based on transition path sampling and likelihood maximization. It requires fewer trajectories than a single iteration of existing procedures, and it applies to both low and high friction dynamics. The new method screens a set of candidate collective variables for a good reaction coordinate that depends on a few relevant variables. The Bayesian information criterion determines whether additional variables significantly improve the reaction coordinate. Additionally, we present an advantageous transition path sampling algorithm and an algorithm to generate the most likely transition path in the space of collective variables. The method is demonstrated on two systems: a bistable model potential energy surface and nucleation in the Ising model. For the Ising model of nucleation, we quantify for the first time the role of nuclei surface area in the nucleation reaction coordinate. Surprisingly, increased surface area increases the stability of nuclei in two dimensions but decreases nuclei stability in three dimensions.
349 citations
TL;DR: In this paper, a complete 6-dimensional potential energy surface for the benzene dimer obtained using symmetry-adapted perturbation theory (SAPT) of intermolecular interactions based on Kohn−Sham's description of monomers is presented.
Abstract: We present a complete 6-dimensional potential energy surface for the benzene dimer obtained using symmetry-adapted perturbation theory (SAPT) of intermolecular interactions based on Kohn−Sham's description of monomers. Ab initio calculations were performed for 491 dimer geometries in a triple-ζ-quality basis set supplemented by bond functions. An accurate analytic fit to the ab initio results has been developed and low-energy stationary points on the potential energy surface have been found. We have determined that there are three minima on the surface. Two of them, the tilted T-shape and the parallel-displaced, are nearly isoenergetic with interaction energies of −2.77 and −2.74 kcal/mol, respectively. The third minimum, a twisted edge-to-edge conformation, is significantly less attractive, with the interaction energy equal to −1.82 kcal/mol. Both the T-shape and sandwich geometries, sometimes assumed to be minima, are shown to be only saddle points. The potential energy surface is extremely flat between...
344 citations
TL;DR: The highly sensitive H atom Rydberg tagging time-of-flight method is used to conduct a crossed molecular beam scattering study of the F + H2 → HF + H reaction with full quantum-state resolution, finding pronounced forward-scattered HF products in the v′ = 2 vibrational state.
Abstract: Reaction resonances, or transiently stabilized transition-state structures, have proven highly challenging to capture experimentally. Here, we used the highly sensitive H atom Rydberg tagging time-of-flight method to conduct a crossed molecular beam scattering study of the F + H2 --> HF + H reaction with full quantum-state resolution. Pronounced forward-scattered HF products in the v' = 2 vibrational state were clearly observed at a collision energy of 0.52 kcal/mol; this was attributed to both the ground and the first excited Feshbach resonances trapped in the peculiar HF(v' = 3)-H' vibrationally adiabatic potential, with substantial enhancement by constructive interference between the two resonances.
273 citations
TL;DR: Energy profiles of the CO insertion into Li-H and Li-CH3 bonds were found to be very similar, especially around the structures where the Li atom is not directly connected with the methyl group, which indicates little effects of alkyl substitution on the reaction route topology.
Abstract: Global reaction route mapping of equilibrium structures, transition structures, and their connections on potential energy surface (PES) has been done for MCHO (M = H, Li, Na, Al, Cu) and HCO2M (M = H, Li). A one-after-another technique based on the scaled hypersphere search method has been successfully applied to exploring unknown chemical structures, transition structures, and reaction pathways for organometallic systems. Upon metal substitution, considerable changes of stable structures, reaction pathways, and relative heights of transition structures have been discovered, though some features are similar among the analogues. Al and Cu atoms were found to behave as very strong scissors to cut the CO double bond in MCHO. Energy profiles of the CO insertion into Li-H and Li-CH3 bonds were found to be very similar, especially around the structures where the Li atom is not directly connected with the methyl group, which indicates little effects of alkyl substitution on the reaction route topology.
250 citations
11 Dec 2006
TL;DR: In this article, a complete set of V-T relaxation rates and dissociation coefficients for the system O-O2 have been obtained by using quasiclassical trajectories on the Varandas and Pais potential energy surface.
Abstract: A complete set of V–T (vibration–translation) relaxation rates and of dissociation coefficients for the system O–O2 have been obtained by using quasiclassical trajectories on the Varandas and Pais potential energy surface. The results, averaged on a Boltzmann rotational distribution, cover the whole range of the vibrational ladder and are reproduced in closed form ready to be implemented in state-to-state kinetic models. The accuracy of the results has been tested by comparing them with available experimental and theoretical values (ASI-CAST project is acknowledged).
239 citations
TL;DR: Calculations using the CASSCF based CT method with single and double operators for the potential energy curves for water and nitrogen molecules, the BeH(2) insertion reaction, and hydrogen fluoride and boron hydride bond breaking consistently yield quantitative accuracies typical of equilibrium region coupled cluster theory, but across all geometries, and better than obtained with multireference perturbation theory.
Abstract: We propose a theory to describe dynamic correlations in bonding situations where there is also significant nondynamic character. We call this the canonical transformation (CT) theory. When combined with a suitable description of nondynamic correlation, such as given by a complete-active-space self-consistent Field (CASSCF) or density matrix renormalization group wave function, it provides a theory to describe bonding situations across the entire potential energy surface with quantitative accuracy for both dynamic and nondynamic correlation. The canonical transformation theory uses a unitary exponential ansatz, is size consistent, and has a computational cost of the same order as a single-reference coupled cluster theory with the same level of excitations. Calculations using the CASSCF based CT method with single and double operators for the potential energy curves for water and nitrogen molecules, the BeH_2 insertion reaction, and hydrogen fluoride and boron hydride bond breaking, consistently yield quantitative accuracies typical of equilibrium region coupled cluster theory, but across all geometries, and better than obtained with multireference perturbation theory.
239 citations
TL;DR: The net result is that the BH&H functional, presumably due to fortuitous cancellation of errors, provides a pragmatic, computationally efficient quantum mechanical tool for the study of large π‐stacked systems such as DNA.
Abstract: The suitability of a hybrid density functional to qualitatively reproduce geometric and energetic details of parallel pi-stacked aromatic complexes is presented. The hybrid functional includes an ad hoc mixture of half the exact (HF) exchange with half of the uniform electron gas exchange, plus Lee, Yang, and Parr's expression for correlation energy. This functional, in combination with polarized, diffuse basis sets, gives a binding energy for the parallel-displaced benzene dimer in good agreement with the best available high-level calculations reported in the literature, and qualitatively reproduces the local MP2 potential energy surface of the parallel-displaced benzene dimer. This method was further critically compared to high-level calculations recently reported in the literature for a range of pi-stacked complexes, including monosubstituted benzene-benzene dimers, along with DNA and RNA bases, and generally agrees with MP2 and/or CCSD(T) results to within +/-2 kJ mol(-1). We also show that the resulting BH&H binding energy is closely related to the electron density in the intermolecular region. The net result is that the BH&H functional, presumably due to fortuitous cancellation of errors, provides a pragmatic, computationally efficient quantum mechanical tool for the study of large pi-stacked systems such as DNA.
235 citations
TL;DR: Comparisons with experiment suggest that the present dynamically corrected VRC-TST approach provides quantitatively accurate predictions for the capture rate, and the present approach should be applicable to a wide array of radical-radical combination reactions.
Abstract: An ab initio transition state theory based procedure for accurately predicting the combination kinetics of two alkyl radicals is described. This procedure employs direct evaluations of the orientation dependent interaction energies at the CASPT2/cc-pvdz level within variable reaction coordinate transition state theory (VRC-TST). One-dimensional corrections to these energies are obtained from CAS+1+2/aug-cc-pvtz calculations for CH3
+ CH3 along its combination reaction path. Direct CAS+1+2/aug-cc-pvtz calculations demonstrate that, at least for the purpose of predicting the kinetics, the corrected CASPT2/cc-pvdz potential energy surface is an accurate approximation to the CAS+1+2/aug-cc-pvtz surface. Furthermore, direct trajectory simulations, performed at the B3LYP/6-31G* level, indicate that there is little local recrossing of the optimal VRC transition state dividing surface. The corrected CASPT2/cc-pvdz potential is employed in obtaining direct VRC-TST kinetic predictions for the self and cross combinations of methyl, ethyl, iso-propyl, and tert-butyl radicals. Comparisons with experiment suggest that the present dynamically corrected VRC-TST approach provides quantitatively accurate predictions for the capture rate. Each additional methyl substituent adjacent to a radical site is found to reduce the rate coefficient by about a factor of two. In each instance, the rate coefficients are predicted to decrease quite substantially with increasing temperature, with the more sterically hindered reactants having a more rapid decrease. The simple geometric mean rule, relating the capture rate for the cross reaction to those for the self-reactions, is in remarkably good agreement with the more detailed predictions. With suitable generalizations the present approach should be applicable to a wide array of radical–radical combination reactions.
200 citations
TL;DR: It is shown that the coherent switching with decay of mixing (CSDM) non-BO trajectory method provides a robust and accurate way to extend molecular dynamics to treat electronically nonadiabatic chemistry for all three kinds of nonadiABatic interactions, and it is recommended for molecular dynamics simulations involving nonradiative electronic state changes.
Abstract: Electronically nonadiabatic or non-Born−Oppenheimer (non-BO) chemical processes (photodissociation, charge-transfer, etc.) involve a nonradiative change in the electronic state of the system. Molecular dynamics simulations typically treat nuclei as moving classically on a single adiabatic potential energy surface, and these techniques are not immediately generalizable to non-BO systems due to the inherently quantum mechanical nature of electronic transitions. Here we generalize the concept of a single-surface molecular dynamics trajectory to that of a coupled-surface non-BO trajectory that evolves “semiclassically” under the influence of two or more electronic states and their couplings. Five non-BO trajectory methods are discussed. Next, we summarize the results of a series of systematic studies using a database of accurate quantum mechanical reaction probabilities and internal energy distributions for several six-dimensional model bimolecular scattering collisions. The test set includes three kinds of p...
195 citations
TL;DR: In this article, a discrete path sampling approach is used to obtain phenomenological two-state rate for a set of local minima with a particular structural motif. But the authors do not consider the transition states that link individual local minimima, and evaluate rate constants for the corresponding elementary rearrangements.
Abstract: The stationary points of a potential energy surface provide a convenient framework for coarse-graining calculations of thermodynamics and kinetics. Thermodynamic properties can be extracted from a database of local minima using the superposition approach, where the total partition function is written as a sum over the contributions from each minimum. To analyse kinetics, we must also consider the transition states that link individual local minima, and evaluate rate constants for the corresponding elementary rearrangements. For small molecules the assignment of separate thermodynamic quantities, such as free energies, to individual isomers, and the notion of isomerisation rates between these structures, is usually straightforward. However, for larger systems the experimental states of interest generally correspond to sets of local minima with some common feature, such as a particular structural motif. This review focuses upon the discrete path sampling approach to obtaining phenomenological two-state rate...
195 citations
TL;DR: Stepping between the local minima of V provides powerful methods for locating the global potential energy minimum, and for calculating global thermodynamic properties, and when the transition states that link local minata are also sampled the authors can exploit statistical rate theory to obtain insight into global dynamics and rare events.
Abstract: Familiar concepts for small molecules may require reinterpretation for larger systems. For example, rearrangements between geometrical isomers are usually considered in terms of transitions between the corresponding local minima on the underlying potential energy surface, V. However, transitions between bulk phases such as solid and liquid, or between the denatured and native states of a protein, are normally addressed in terms of free energy minima. To reestablish a connection with the potential energy surface we must think in terms of representative samples of local minima of V, from which a free energy surface is projected by averaging over most of the coordinates. The present contribution outlines how this connection can be developed into a tool for quantitative calculations. In particular, stepping between the local minima of V provides powerful methods for locating the global potential energy minimum, and for calculating global thermodynamic properties. When the transition states that link local minima are also sampled we can exploit statistical rate theory to obtain insight into global dynamics and rare events. Visualizing the potential energy landscape helps to explain how the network of local minima and transition states determines properties such as heat capacity features, which signify transitions between free energy minima. The organization of the landscape also reveals how certain systems can reliably locate particular structures on the experimental time scale from among an exponentially large number of local minima. Such directed searches not only enable proteins to overcome Levinthal's paradox but may also underlie the formation of "magic numbers" in molecular beams, the self-assembly of macromolecular structures, and crystallization.
TL;DR: The results show very good agreement with density functional theory calculations, reproducing the individual CSOV energy contributions for a given interaction as well as the B3LYP total interaction energies with errors below kBT at room temperature.
Abstract: Total intermolecular interaction energies are determined with a first version of the Gaussian electrostatic model (GEM-0), a force field based on a density fitting approach using s-type Gaussian functions. The total interaction energy is computed in the spirit of the sum of interacting fragment ab initio (SIBFA) force field by separately evaluating each one of its components: electrostatic (Coulomb), exchange repulsion, polarization, and charge transfer intermolecular interaction energies, in order to reproduce reference constrained space orbital variation (CSOV) energy decomposition calculations at the B3LYP/aug-cc-pVTZ level. The use of an auxiliary basis set restricted to spherical Gaussian functions facilitates the rotation of the fitted densities of rigid fragments and enables a fast and accurate density fitting evaluation of Coulomb and exchange-repulsion energy, the latter using the overlap model introduced by Wheatley and Price [Mol. Phys. 69, 50718 (1990)]. The SIBFA energy scheme for polarization and charge transfer has been implemented using the electric fields and electrostatic potentials generated by the fitted densities. GEM-0 has been tested on ten stationary points of the water dimer potential energy surface and on three water clusters (n=16,20,64). The results show very good agreement with density functional theory calculations, reproducing the individual CSOV energy contributions for a given interaction as well as the B3LYP total interaction energies with errors below kBT at room temperature. Preliminary results for Coulomb and exchange-repulsion energies of metal cation complexes and coupled cluster singles doubles electron densities are discussed.
TL;DR: The excited-state component in the nanosecond range detected in the time-resolved photoelectron spectrum is proposed to be a triplet (pi,pi) state formed after intersystem crossing from the singlet (n,pi), using the CAS-PT2//CASSCF level results to interpret the photophysics.
Abstract: The decay paths on the singlet excited-state surface of 9H-adenine and the associated energy barriers have been calculated at the CAS−PT2//CASSCF level. There are three fundamental paths for the photophysics: two paths for the 1Lb state which are virtually barrierless at the present level of theory and correspond to formation of the (n,π*) intermediate and direct decay to the ground state and a third path for ground-state decay of the (n,π*) state with an activation barrier of approximately 0.1 eV. The 1La state, which has the largest oscillator strength, either decays directly to the ground state or contributes indirectly to the excited-state lifetime by populating the two other states. The results are used to interpret the photophysics in terms of an excited-state plateau for the 1Lb state that corresponds to the short-lived excited-state component (approximately 0.1 ps) and a well (i.e., a proper minimum) for the (n,π*) state that gives rise to the long component (1 ps or more). The direct decay to th...
TL;DR: It is shown that models neglecting the spatial inhomogeneity and thus the local structure serve as useful empirical tools for particular purposes, e.g. for understanding the main features of the complex redox properties that are so crucial for many applications of these oxides.
Abstract: The present review focuses on links between structure, energetics and ion transport in oxygen-deficient perovskite oxides, ABO3−δ. The perfect long-range order, convenient for interpretations of the structure and properties of ordered materials, is evidently not present in disordered materials and highly defective perovskite oxides are spatially inhomogeneous on an intermediate length scale. Although this makes a fundamental description of these and other disordered materials very difficult, it is becoming increasingly clear that this complexity is often essential for the functional properties. In the present review we advocate a potential energy barrier description of the disordered state in which the possible local (or inherent) structures are seen to correspond to separate local minima on the potential energy surface. We interpret the average structure observed experimentally at any temperature as a time and spatial average of the different local structures which are energetically accessible. The local structure is largely affected by preferences for certain polyhedron coordinations and the oxidation state stability of the transition metals, and the strong long-range electrostatic interactions present in non-stoichiometric oxides imply that only a small fraction of the local energy minima on the potential energy surface are accessible at most temperatures. We will show that models neglecting the spatial inhomogeneity and thus the local structure serve as useful empirical tools for particular purposes, e.g. for understanding the main features of the complex redox properties that are so crucial for many applications of these oxides. The short-range order is on the other hand central for understanding ionic transport. Oxide ion transport involves the transformation of one energetically accessible local structure into another. Thus, strongly correlated transport mechanisms are expected; in addition to the movement of the oxygen ions giving rise to the transport, other ions are involved and even the A and B atoms move appreciably in a cooperative fashion along the transition path. Such strongly correlated or collective ionic migration mechanisms should be considered for fast oxide ion conductors in general and in particular for systems forming superstructures at low temperatures. Structural criteria for fast ion conduction are discussed. A high density of low-lying local energy minima is certainly a prerequisite and for perovskite-related A2B2O5 oxides, those containing B atoms that have energetic preference for tetrahedral coordination geometry are especially promising.
TL;DR: Ab initio calculations of portions of the C2H5O potential energy surface critical to the title reaction are presented and it is found that a significant fraction of the total rate coefficient is due to the formation of vinyl alcohol above this temperature.
Abstract: Ab initio calculations of portions of the C2H5O potential energy surface critical to the title reaction are presented. These calculations are based on QCISD geometries and frequencies and RQCISD(T) energies extrapolated to the complete-basis-set limit. Rate coefficients for the reaction of C2H4 with OH are calculated using this surface and the two transition-state model of Greenwald and co-workers [J. Phys. Chem. A 2005, 109, 6031] for the association of OH with C2H4. The present calculations reproduce most of the experimental data, including the temperature and pressure dependence of the rate coefficients, with only a small (0.4 kcal/mol) adjustment to the energy barrier for direct hydrogen abstraction. We confirm the importance of this channel above 800 K and find that a significant fraction of the total rate coefficient (∼10%) is due to the formation of vinyl alcohol above this temperature. Calculations of the vinyl alcohol channel are consistent with the recent observation of this molecule in low-pres...
TL;DR: Excited state reaction paths and the corresponding energy profiles of salicylic acid have been determined with the CC2 method, which is a simplified version of singles-and-doubles coupled cluster theory.
Abstract: Excited state reaction paths and the corresponding energy profiles of salicylic acid have been determined with the CC2 method, which is a simplified version of singles-and-doubles coupled cluster theory. At crucial points of the potential energy hypersurfaces, single-point energy calculations have been performed with the CASPT2 method (second-order perturbation theory based on the complete active space self-consistent field reference). Hydrogen transfer along the intramolecular hydrogen bond as well as torsion and pyramidization of the carboxy group have been identified as the most relevant photochemical reaction coordinates. The keto-type planar S1 state reached by barrierless intramolecular hydrogen transfer represents a local minimum of the S1 energy surface, which is separated by a very low barrier from a reaction path leading to a low-lying S1–S0 conical intersection via torsion and pyramidization of the carboxy group. The S1–S0 conical intersection, which occurs for perpendicular geometry of the carboxy group, is a pure biradical. From the conical intersection, a barrierless reaction path steers the system back to the two known minima of the S0 potential energy surface (rotamer I, rotamer II). A novel structure, 7-oxa-bicyclo[4.2.0]octa-1(6),2,4-triene-8,8-diol, has been identified as a possible transient intermediate in the photophysics of salicylic acid.
TL;DR: New results for the water dimer equilibrium constant K(p)(T) are presented, using a flexible potential energy surface fitted to spectroscopical data and an analysis of the convergence of the partition function reveals that quasi-bound states are likely to contribute to the equilibrium constant.
Abstract: We present new results for the water dimer equilibrium constant Kp(T) in the range 190−390 K, using a flexible potential energy surface fitted to spectroscopical data. The increased numerical complexity due to explicit consideration of the monomer vibrations is handled via an adiabatic (6 + 6)d decoupling between intra- and intermolecular modes. The convergence of the canonical partition function of the dimer is ensured by computing all energy levels up to dissociation for total angular momentum values J = 0−5 and using an extrapolation scheme to higher values. The newly calculated values for Kp(T) are in very good agreement with available experimental data at room temperature. At higher temperatures, an analysis of the convergence of the partition function reveals that quasi-bound states are likely to contribute to the equilibrium constant. Additional thermodynamical quantities (ΔG, ΔH, ΔS, and Cp) have also been determined and fit to quadratic expressions a + bT + cT2.
TL;DR: A full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) are reported for the water dimer, (H2O)2, and their geometrical and vibrational properties are presented and compared with available previous theoretical work.
Abstract: A full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) are reported for the water dimer, (H2O)2. The CCSD(T)-PES is a very precise fit to 19 805 ab initio energies obtained with the coupled-cluster (CCSD(T)) method, using an aug-cc-pVTZ basis. The standard counterpoise correction was applied to approximately eliminate basis set superposition errors. The fit is based on an approach that incorporates the permutational symmetry of identical atoms [Huang, X.; Braams, B.; Bowman, J. M. J. Chem. Phys. 2005, 122, 044308]. The DMS is a fit to the dipole moment obtained with Moller−Plesset (MP2) theory, using an aug-cc-pVTZ basis. The PES has an RMS fitting error of 31 cm-1 for energies below 20 000 cm-1, relative to the global minimum. This surface can describe various internal floppy motions, including various monomer inversions, and isomerization pathways. Ten characteristic stationary points have been located on the surface, four of which are transition structures and the r...
TL;DR: In this paper, the rotational (de)excitation of CO by ground state para- and ortho-H_2 is obtained using quantum scattering calculations for collision energies between 1 and 520 cm^{-1}.
Abstract: Cross sections for the rotational (de)excitation of CO by ground state para- and ortho-H_2 are obtained using quantum scattering calculations for collision energies between 1 and 520 cm^{-1}. A new CO-H_2 potential energy surface is employed and its quality is assessed by comparison with explicitly correlated CCSD(T)-R12 calculations. Rate constants for rotational levels of CO up to 5 and temperatures in the range 5-70 K are deduced. The new potential is found to have a strong influence on the resonance structure of the cross sections at very low collision energies. As a result, the present rates at 10 K differ by up to 50% with those obtained by \citet{flower01} on a previous, less accurate, potential energy surface.
TL;DR: In this article, the rotational (de)excitation of CO by ground state para-and ortho-H 2 is obtained using quantum scattering calculations for collision energies between 1 and 520 cm -1.
Abstract: Cross sections for the rotational (de)excitation of CO by ground state para- and ortho-H 2 are obtained using quantum scattering calculations for collision energies between 1 and 520 cm -1 . A new CO-H 2 potential energy surface is employed and its quality is assessed by comparison with explicitly correlated CCSD(T)-R12 calculations. Rate constants for rotational levels of CO up to 5 and temperatures in the range 5-70 K are deduced. The new potential is found to have a strong influence on the resonance structure of the cross sections at very low collision energies. As a result, the present rates at 10 K differ by up to 50% with those obtained by Flower (2001) on a previous, less accurate, potential energy surface.
TL;DR: A combined experimental and theoretical study on the dynamics of the N(2D) + H2 insertion reaction at a collision energy of 15.9 kJ mol(-1) to compare the accurate QM differential cross sections with those obtained by two approximate methods, namely, the widely used quasiclassical trajectory calculations and a rigorous statistical method based on the coupled-channel theory.
Abstract: In this paper, we report a combined experimental and theoretical study on the dynamics of the N(2D) + H 2 insertion reaction at a collision energy of 15.9 kJ mol -1 . Product angular and velocity distributions have been obtained in crossed beam experiments and simulated by using the results of quantum mechanical (QM) scattering calculations on the accurate ab initio potential energy surface (PES) of Pederson et al. (J. Chem. Phys. 1999, 110, 9091). Since the QM calculations indicate that there is a significant coupling between the product angular and translational energy distributions, such a coupling has been explicitly included in the simulation of the experimental results. The very good agreement between experiment and QM calculations sustains the accuracy of the NH 2 ab initio ground state PES. We also take the opportunity to compare the accurate QM differential cross sections with those obtained by two approximate methods, namely, the widely used quasiclassical trajectory calculations and a rigorous statistical method based on the coupled-channel theory.
TL;DR: The present work describes in detail the various convergence test performed to investigate the accuracy of the calculations at each step and demonstrates the predictive power of the present calculations.
Abstract: Calculations for the cumulative reaction probability N(E) (for J=0) and the thermal rate constant k(T) of the H+CH4→H2+CH3 reaction are presented. Accurate electronic structure calculations and a converged Shepard-interpolation approach are used to construct a potential energy surface which is specifically designed to allow the precise calculation of k(T) and N(E). Accurate quantum dynamics calculations employing flux correlation functions and multiconfigurational time-dependent Hartree wave packet propagation compute N(E) and k(T) based on this potential energy surface. The present work describes in detail the various convergence test performed to investigate the accuracy of the calculations at each step. These tests demonstrate the predictive power of the present calculations. In addition, approximate approaches for reaction rate calculations are discussed. A quite accurate approximation can be obtained from a potential energy surface which includes only interpolation points on the minimum energy path.
TL;DR: In this article, a 2D SiO-He potential energy surface is computed by means of highly correlated ab initio calculations, and rate coefficients for rotational (de)excitation of SiO by collision with He are obtained from close-coupling quantum scattering calculations of inelastic cross sections.
Abstract: Context. Within shocked regions of the interstellar medium and circumstellar environment of AGB stars the proper modelling of SiO line emission through non-LTE radiative transfer calculations requires accurate values of collisional rate coefficients. Aims. The present study focuses on the transitions among the rotational levels of the SiO molecule in its ground vibrational state induced by collision with He. The H 2 molecule being the main colliding partner for the astrophysical regions of interest, the collisional process between SiO and para-H 2 (j = 0) is also investigated in an approximated way. Methods. A new 2D SiO-He potential energy surface is computed by means of highly correlated ab initio calculations. Collisional rate coefficients corresponding to the pure rotational (de)excitation of SiO by collision with He are obtained from close-coupling quantum scattering calculations of inelastic cross sections. The SiO-He potential energy surface is also employed to compute rate coefficients for the rotational (de)excitation of SiO by collision with para-H 2 (j = 0). Results. Rate coefficients for rotational levels up to j = 26 and kinetic temperatures in the range 10-300 K are obtained for the SiO-He colliding system. The large asymmetry of the SiO-He potential energy surface induces a propensity rule that favours odd Δj transitions over even Δj. The estimated values of the SiO-para-H 2 (j = 0) rate coefficients are compared with those of Turner et al. (1992) for the twenty first rotational levels. As a result of significant differences between the SiO-He interaction potentials employed in the two studies, the rate coefficients are found to differ by a factor 2.5-5 for the main rotational transitions, whatever the temperature range.
TL;DR: Excited-state potential energy surface (PES) characterization is carried out at the CASSCF and MRSDCI levels, followed by ab initio dynamics simulation of excited-state intramolecular proton transfer (ESIPT) on the S2(pipi*) state in malonaldehyde.
Abstract: Excited-state potential energy surface (PES) characterization is carried out at the CASSCF and MRSDCI levels, followed by ab initio dynamics simulation of excited-state intramolecular proton transfer (ESIPT) on the S2(ππ*) state in malonaldehyde. The proton-transfer transition state lies close to an S2/S1 conical intersection, leading to substantial coupling of proton transfer with electronic relaxation. Proton exchange proceeds freely on S2, but its duration is limited by competition with twisting out of the molecular plane. This rotamerization pathway leads to an intersection of the three lowest singlet states, providing the first detailed report of ab initio dynamics around a three-state intersection (3SI). There is a significant energy barrier to ESIPT on S1, and further pyramidalization of the twisted structure leads to the minimal energy S1/S0 intersection and energetic terminal point of excited-state dynamics. Kinetics and additional mechanistic details of these pathways are discussed. Significant ...
TL;DR: It is found that superradiant emission, that is, an enhancement of the fluorescence rate in the dimer, is observed only in the case of weak and medium coupling.
Abstract: We investigated the spectroscopic properties of a series of four bistriarylamine donor−π−bridge-donor D−π−D compounds (dimers), composed of two asymmetric triarylamine chromophores (monomers). UV/vis, fluorescence, and transient absorption spectra were recorded and compared with those of the corresponding D-π monomers. Bilinear Lippert−Mataga plots indicate a major molecular reorganization of the excited state in polar media for all compounds. The excited states of the dimers are described as mixed-valence states that show, depending on the chemical nature of the π bridge, a varying amount of interactions (couplings). We found that superradiant emission, that is, an enhancement of the fluorescence rate in the dimer, is observed only in the case of weak and medium coupling. Whether the first excited-state potential energy surface of the dimers is described by single minimum or a double minimum potential depends on the solvent polarity and the electronic coupling. In the latter case, the dimer relaxes in a ...
TL;DR: A full-dimensional, ab initio based potential energy surface (PES) for CH(5)(+), which can describe dissociation is reported, to validate the applicability of the PES for dynamics studies as well as to test a simple classical statistical prediction of the branching ratio of the dissociation products.
Abstract: A full-dimensional, ab initio based potential energy surface (PES) for CH5+, which can describe dissociation is reported. The PES is a precise fit to 36173 coupled-cluster [CCSD(T)] calculations of...
TL;DR: Compared with this third-row transition metal system with the first-row and second-row congeners, Cu+ and Ag+.
Abstract: The potential energy surface for activation of methane by the third-row transition metal cation, Au+, is studied experimentally by examining the kinetic energy dependence of this reaction using guided ion beam tandem mass spectrometry. A flow tube ion source produces Au+ primarily in its 1S0 (5d10) electronic ground state level but with some 3D (and perhaps higher lying) excited states that can be completely removed by a suitable quenching gas (N2O). Au+ (1S0) reacts with methane by endothermic dehydrogenation to form AuCH2+ as well as C-H bond cleavage to yield AuH+ and AuCH3+. The kinetic energy dependences of the cross sections for these endothermic reactions are analyzed to give 0 K bond dissociation energies (in eV) of D0(Au+ - CH2) = 3.70 +/- 0.07 and D0(Au+ -CH3) = 2.17 +/- 0.24. Ab initio calculations at the B3LYPHW + /6-311++G(3df,3p) level performed here show good agreement with the experimental bond energies and previous theoretical values available. Theory also provides the electronic structures of the product species as well as intermediates and transition states along the reactive potential energy surface. Surprisingly, the dehydrogenation reaction does not appear to involve an oxidative addition mechanism. We also compare this third-row transition metal system with the first-row and second-row congeners, Cu+ and Ag+. Differences in thermochemistry can be explained by the lanthanide contraction and relativistic effects that alter the relative size of the valence s and d orbitals.
TL;DR: In this article, the authors investigate how the prior chemisorption of an H atom on graphite influences the adsorption properties of a second H atom at neighboring sites of the same carbon ring of the surface.
TL;DR: An ab initio-based global potential energy surface for H+CH4 that describes the abstraction and exchange reactions and is a fit to 20,728 electronic energies calculated using the partially spin-restricted coupled-cluster method with a moderately large basis (aug-cc-pVTZ).
Abstract: We report an ab initio-based global potential energy surface for H+CH4 that describes the abstraction and exchange reactions. The PES, which is invariant with respect to any permutation of five H atoms, is a fit to 20,728 electronic energies calculated using the partially spin-restricted coupled-cluster method (RCCSD(T)) with a moderately large basis (aug-cc-pVTZ). A first set of quasiclassical trajectory calculations using this PES are reported for the H+CD4-->HD+CD3 reaction at collision energies of 0.65 and 1.52 eV and are compared to experiment and recent direct dynamics calculations done with density functional theory.
TL;DR: Direct molecular orbital-molecular dynamics (MO-MD) calculation was applied to diffusion processes of the Li atom on a model surface of amorphous carbon and compared with the diffusion mechanism of Li+ ion, which shows that theLi atom vibrates around the initial position below 250 K, and it moves above 300 K.
Abstract: Direct molecular orbital-molecular dynamics (MO-MD) calculation was applied to diffusion processes of the Li atom on a model surface of amorphous carbon and compared with the diffusion mechanism of Li+ ion. A carbon sheet composed of C96H24 was used as the model surface. The total energy and energy gradient on the full dimensional potential energy surface of the LiC96H24 system were calculated at each time step in the trajectory calculation. The optimized structure, where the Li atom is located at the center of mass of the model surface, was used as the initial structure at time zero. Simulation temperatures were chosen in the range of 200-1250 K. The dynamics calculations showed that the Li atom vibrates around the initial position below 250 K, and it moves above 300 K. At middle temperature, the Li atom translates freely on the surface. At higher temperature (1000 K), the Li atom moves from the center to edge region of the model surface and is trapped in the edge. The activation energy calculated for the Li atom is larger than that for the Li+ ion. This difference is due to the fact that the Li atom diffuses together with an unpaired electron on the carbon surface. The diffusion mechanism of the Li atom was discussed on the basis of the theoretical results.