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Showing papers on "Potential energy surface published in 2005"


Journal ArticleDOI
TL;DR: In this paper, the potential energy surface of curcumin [1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione] was explored with the DFT correlation functional B3LYP method using 6-311G* basis.
Abstract: The potential energy surface of curcumin [1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione] was explored with the DFT correlation functional B3LYP method using 6-311G* basis. The single-point calculations were performed at levels up to B3LYP/6-311++G**//B3LYP/6-311G*. All isomers were located and relative energies determined. According to the calculation the planar enol form is more stable than the nonplanar diketo form. The results of the optimized molecular structure are presented and compared with the experimental X-ray diffraction. In addition, harmonic vibrational frequencies of the molecule were evaluated theoretically using B3LYP density functional methods. The computed vibrational frequencies were used to determine the types of molecular motions associated with each of the experimental bands observed. Our vibrational data show that in both the solid state and in all studied solutions curcumin exists in the enol form. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

361 citations


Journal ArticleDOI
15 Apr 2005-Science
TL;DR: It is shown that, to first order in time, the dynamics are inertial, and constraints on the shape and curvature of the transition-state potential energy surface are placed, pointing toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.
Abstract: The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.

315 citations


Journal ArticleDOI
TL;DR: Full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) are reported for H(5)O(2) (+ and the PES dissociates correctly (and symmetrically) to H(2), with D(e)=11 923.8 cm(-1).
Abstract: Full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) are reported for H5O2+. Tens of thousands of coupled-cluster [CCSD(T)] and second-order Moller-Plesset (MP2) calculations of electronic energies, using aug-cc-pVTZ basis, were done. The energies were fit very precisely in terms of all the internuclear distances, using standard least-square procedures, however, with a fitting basis that satisfies permutational symmetry with respect to like atoms. The H5O2+ PES is a fit to 48 189 CCSD(T) energies, containing 7962 polynomial coefficients. The PES has a rms fitting error of 34.9 cm−1 for the entire data set up to 110 000 cm−1. This surface can describe various internal floppy motions, including the H atom exchanges, monomer inversions, and monomer torsions. First- and higher-order saddle points have been located on the surface and compared with available previous theoretical work. In addition, the PES dissociates correctly (and symmetrically) to H2O+H3O+, with De=11 923....

261 citations


Journal ArticleDOI
TL;DR: A computational study that locates the global minima in the potential energy surface of Ti(n)O2n clusters with n = 1-15 and reports novel structures that provide the basis for further computational studies of the effect of nanostructure on adsorption, photochemistry, and nucleation of this material.
Abstract: The effect of the nanostructure on the photochemistry of TiO2 is an active field of research owing to its applications in photocatalysis and photovoltaics. Despite this interest, little is known of the structure of small particles of this oxide with sizes at the nanometer length scale. Here we present a computational study that locates the global minima in the potential energy surface of TinO2n clusters with n = 1−15. The search procedure does not refer to any of the known TiO2 polymorphs, and is based on a novel combination of simulated annealing and Monte Carlo basin hopping simulations, together with genetic algorithm techniques, with the energy calculated by means of an interatomic potential. The application of several different methods increases our confidence of having located the global minimum. The stable structures are then refined by means of density functional theory calculations. The results from the two techniques are similar, although the methods based on interatomic potentials are unable to...

209 citations


Journal ArticleDOI
TL;DR: The edge-to-face interactions for either axially or facially substituted benzenes are investigated by using ab initio calculations and the dispersion energy is the most dominating interaction in both axial and facial substitutions.
Abstract: The edge-to-face interactions for either axially or facially substituted benzenes are investigated by using ab initio calculations. The predicted maximum energy difference between substituted and unsubstituted systems is ∼0.7 kcal/mol (∼1.2 kcal/mol if substituents are on both axially and facially substituted positions). In the case of axially substituted aromatic systems, the electron density at the para position is an important stabilizing factor, and thus the stabilization/destabilization by substitution is highly correlated to the electrostatic energy. This results in its subsequent correlation with the polarization and charge transfer. Thus, the stabilization/destabilization by substitution is represented by the sum of electrostatic energy and induction energy. On the other hand, the facially substituted aromatic system depends on not only the electron-donating ability responsible for the electrostatic energy but also the dispersion interaction and exchange repulsion. Although the dispersion energy i...

179 citations


Journal ArticleDOI
TL;DR: It is shown that a one-dimensional reaction coordinate can be constructed even when the diffusion anisotropy is arbitrary, and the rate constant is identical to that predicted by the multidimensional Kramers-Langer theory.
Abstract: For multidimensional activated rate processes controlled by diffusive crossing of a saddle point region, we show that a one-dimensional reaction coordinate can be constructed even when the diffusion anisotropy is arbitrary. The rate constant, found using the potential of mean force along this coordinate, is identical to that predicted by the multidimensional Kramers–Langer theory. This reaction coordinate minimizes the one-dimensional rate constant obtained using a trial reaction coordinate and is orthogonal to the stochastic separatrix, the transition state that separates reactants from products.

178 citations


Journal ArticleDOI
TL;DR: A collective coordinate response for the solvent effect is constructed by identifying the main electrostatic field and gradient components contributing to the line shape, which allows a realistic stochastic Liouville equation simulation of the line shapes which is not restricted to Gaussian frequency fluctuations.
Abstract: An ab initio MP2 vibrational Hamiltonian of HOD in an external electrostatic potential parametrized by the electric field and its gradient-tensor is constructed. By combining it with the fluctuating electric field induced by the D2O solvent obtained from molecular dynamics simulations, we calculate the infrared absorption of the O-H stretch. The resulting solvent shift and infrared line shape for three force fields (TIP4P, SPC/E, and SW) are in good agreement with the experiment. A collective coordinate response for the solvent effect is constructed by identifying the main electrostatic field and gradient components contributing to the line shape. This allows a realistic stochastic Liouville equation simulation of the line shapes which is not restricted to Gaussian frequency fluctuations.

152 citations


Journal ArticleDOI
TL;DR: The identified conical intersections of the pi --> pi excited states of the keto cytosine tautomers are made responsible for the ultrafast decay to the electronic ground states and thus may explain their subpicoseconds lifetimes.
Abstract: The low-lying excited singlet states of the keto, enol, and keto-imine tautomers of cytosine have been investigated employing a combined density functional/multireference configuration interaction (DFT/MRCI) method. Unconstrained geometry optimizations have yielded out-of-plain distorted structures of the pi --> pi and n --> pi excited states of all cytosine forms. For the keto tautomer, the DFT/MRCI adiabatic excitation energy of the pi --> pi state (4.06 eV including zero-point vibrational energy corrections) supports the resonant two-photon ionization (R2PI) spectrum (Nir et al. Phys. Chem. Chem. Phys. 2002, 5, 4780). On its S1 potential energy surface, a conical intersection between the 1pipi state and the electronic ground state has been identified. The barrier height of the reaction along a constrained minimum energy path amounts to merely 0.2 eV above the origin and explains the break-off of the R2PI spectrum. The 1pipi minimum of the enol tautomer is found at considerably higher excitation energies (4.50 eV). Because of significant geometry shifts with respect to the ground state, long vibrational progressions are expected, in accord with experimental observations. For the keto-imine tautomer, a crossing of the 1pipi potential energy surface with the ground-state surface has been found, too. Its n --> pi minimum (3.27 eV) is located well below the conical intersection between the pi --> pi and S0 states, but it will be difficult to observe because of its small transition moment. The identified conical intersections of the pi --> pi excited states of the keto cytosine tautomers are made responsible for the ultrafast decay to the electronic ground states and thus may explain their subpicoseconds lifetimes.

140 citations


Journal ArticleDOI
TL;DR: These quantum theoretical cross sections derived from the ground rovibrational state of H(2) show wiggling structures and an increasing trend for both the reactive charge transfer and the nonreactive charge transfer but a decreasing trend for the reactive noncharge transfer throughout the investigated collision energy range 1.7-2.5 eV.
Abstract: A theoretical investigation on the nonadiabatic processes of the D(+) + H(2) reaction system has been carried out by means of exact three-dimensional nonadiabatic time-dependent wave packet calculations with an extended split operator scheme (XSOS). The diabatic potential energy surface newly constructed by Kamisaka et al. (J. Chem. Phys. 2002, 116, 654) was employed in the calculations. This study provided quantum cross sections for three competing channels of the reactive charge transfer, the nonreactive charge transfer, and the reactive noncharge transfer, which contrasted markedly to many previous quantum theoretical reports on the (DH(2))(+) system restricted to the total angular momentum J = 0. These quantum theoretical cross sections derived from the ground rovibrational state of H(2) show wiggling structures and an increasing trend for both the reactive charge transfer and the nonreactive charge transfer but a decreasing trend for the reactive noncharge transfer throughout the investigated collision energy range 1.7-2.5 eV. The results also show that the channel of the reactive noncharge transfer with the largest cross section is the dominant one. A further investigation of the v-dependent behavior of the probabilities for the three channels revealed an interesting dominant trend for the reactive charge transfer and the nonreactive charge transfer at vibrational excitation v = 4 of H(2). In addition, the comparison between the centrifugal sudden (CS) and exact calculations showed the importance of the Coriolis coupling for the reactive system. The computed quantum cross sections are also compared with the experimental measurement results.

136 citations


Journal ArticleDOI
TL;DR: Ab initio G2M calculations have been performed to investigate the potential energy surface for the reaction of C6H5 with O2, and the most favorable product channel for the decomposition of 10 is C5H5 + CO2, followed by pyranyl + CO and o-benzoquinone + H.
Abstract: Ab initio G2M calculations have been performed to investigate the potential energy surface for the reaction of C6H5 with O2 The reaction is shown to start with an exothermic barrierless addition of O2 to the radical site of C6H5 to produce phenylperoxy (1) and, possibly, 1,2-dioxaspiro[25]octadienyl (dioxiranyl, 8) radicals Next, 1 loses the terminal oxygen atom to yield the phenoxy + O products (3) or rearranges to 8 The dioxiranyl can further isomerize to a seven-member ring 2-oxepinyloxy radical (10), which can give rise to various products including C5H5 + CO2, pyranyl + CO, o-benzoquinone + H, and 2-oxo-2,3-dihydrofuran-4-yl + C2H2 Once 10 is produced, it is unlikely to go back to 8 and 1, because the barriers separating 10 from the products are much lower than the reverse barrier from 10 to 8 Thus, the branching ratio of C6H5O + O against the other products is mostly controlled by the critical transition states between 1 and 3, 1 and 8, and 8 and 10 According to the calculated barriers, the most favorable product channel for the decomposition of 10 is C5H5 + CO2, followed by pyranyl + CO and o-benzoquinone + H Since C6H5O + O and C5H5 + CO2 are expected to be the major primary products of the C6H5 + O2 reaction and thermal decomposition of C6H5O leads to C5H5 + CO, cyclopentadienyl radicals are likely to be the major product of phenyl radical oxidation, and so it results in degradation of the six-member aromatic ring to the five-member cyclopentadienyl ring Future multichannel RRKM calculations of reaction rate constants are required to support these conclusions and to quantify the product branching ratios at various combustion conditions

128 citations


Journal ArticleDOI
TL;DR: In this paper, an ordered silica nanorod with clearly defined nominal tensile stress is constructed to model a structural unit of the stressed crack tip, and three competing hydrolysis reaction pathways are determined, each involving a distinct initiation step.
Abstract: Stress-corrosion of silica by water is studied by exploring the stress-dependent potential energy surface computed quantum mechanically at the level of molecular orbital theory. An ordered silica nanorod with clearly defined nominal tensile stress is constructed to model a structural unit of the stressed crack tip. Three competing hydrolysis reaction pathways are determined, each involving a distinct initiation step. Water dissociation, molecular chemisorption, and direct siloxane bond rupture dominate at low, intermediate, and high stress levels, respectively. A linear stress dependence in the thermodynamic driving force, not commonly considered in the criterion of brittle fracture initiation, is shown to originate from surface relaxation associated with bond rupture. This effect is particularly important in determining the Griffith condition of crack extension for nano-sized systems when spatial accommodation of foreign molecules is involved in the process of bond breaking. The physical origin of the stress dependence of kinetic barrier is revealed by a perturbation analysis of the minimum energy path parametrized by the continuous mechanical stress.

Journal ArticleDOI
TL;DR: An accurate global potential-energy surface (PES) is reported for H5(+) based on more than 100,000 CCSD(T)/aug-cc-pVTZ ab initio energies and has full permutational symmetry with respect to interchange of H atoms and dissociates to H3(+) and H2.
Abstract: An accurate global potential-energy surface (PES) is reported for H5(+) based on more than 100,000 CCSD(T)/aug-cc-pVTZ ab initio energies. This PES has full permutational symmetry with respect to interchange of H atoms and dissociates to H3(+) and H2. Ten known stationary points of H5(+) are characterized and compared to previous ab initio calculations. Quantum diffusion Monte Carlo calculations are performed on the PES to obtain the zero-point energy of H5(+) and the anharmonic dissociation energy (D0) of H5(+) --> H3(+) + H2. The rigorous zero-point state of H4D+ is also calculated and discussed within the context of a strictly classical approach to obtain the branching ratio of the reaction H4D+ --> H3(+) + HD and H2D+ + H2. Such an approach is taken using the PES and critiqued based on the properties of the quantum zero-point state. Finally, a simple procedure for adding the long range-interaction energy is described.

Journal ArticleDOI
TL;DR: Using a recoupling technique with close-coupling spin-free calculations de-excitation rate coefficients are obtained among hyperfine transitions for He colliding with N 2 H +, and a recently determined potential energy surface suitable for scattering calculations is used to investigate rate coefficients for temperatures between 5 and 50 K as mentioned in this paper.
Abstract: Using a recoupling technique with close-coupling spin-free calculations de-excitation rate coefficients are obtained among hyperfine transitions for He colliding with N 2 H + . A recently determined potential energy surface suitable for scattering calculations is used to investigate rate coefficients for temperatures between 5 and 50 K, and for the seven lowest rotational levels of N 2 H + . Fitting functions are provided for the Maxwellian averaged opacity tensors and for the rotational de-excitation collisional rate coefficients. The fitting functions for the opacity tensors can be used to calculate hyperfine (de)-excitation rate coefficients among elastic and inelastic rotational levels, and among the corresponding magnetic sublevels of the hyperfine structure. Certain dynamical approximations are investigated and found to be invalid.

Journal ArticleDOI
Chuanxiu Xu1, Daiqian Xie, Dong H. Zhang, Shi Ying Lin, Hua Guo 
TL;DR: A new global potential-energy surface for the ground electronic state of HO(2)(X(2)A(")) has been developed by three-dimensional cubic spline interpolation of more than 15 000 ab initio points, which were calculated at the multireference configuration-interaction level with Davidson correction using the augmented correlation-consistent polarized valence quadruple zeta basis set.
Abstract: A new global potential-energy surface for the ground electronic state of HO(2)(X(2)A(")) has been developed by three-dimensional cubic spline interpolation of more than 15 000 ab initio points, which were calculated at the multireference configuration-interaction level with Davidson correction using the augmented correlation-consistent polarized valence quadruple zeta basis set. Low-lying vibrational states were obtained in this new potential using the Lanczos method and assigned. The calculated vibrational frequencies are in much better agreement with the available experimental band origins than those obtained from a previous potential. In addition, rate constants for the H+O(2) O + OH reactions were obtained using a wave-packet-based statistical model. Reasonably good agreement with experimental data was obtained. These results demonstrate the accuracy of the potential.

Journal ArticleDOI
TL;DR: The experimental spectra of 2-bromo-4-methyl-phenylamine coincide satisfactorily with those of theoretically constructed bar type spectrograms and several thermodynamic parameters were calculated for the minimum energy conformer at ab initio and DFT level of theories.

Journal ArticleDOI
TL;DR: The results indicate very dynamic structural changes at temperature range relevant to atmospheric chemistry and current experiments and the structures and properties of medium-sized protonated clusters in this temperature range are far from their global minimum cousins.
Abstract: The structure of protonated water clusters H+(H2O)n (n=5–22) are examined by two Monte Carlo methods in conjunction with the OSS2 potential [L. Ojamae, I. Shavitt, and S. J. Singer J. Chem. Phys. 109, 5547 (1998)]. The basin-hopping method is employed to explore the OSS2 potential energy surface and to locate low-energy structures. The topology of the “global minimum,” the most stable low-energy structure, changes from single ring to multiple ring to polyhedral cage as the cluster size grows. The temperature dependence of the cluster geometry is examined by carrying out parallel tempering Monte Carlo simulations. Over the temperature range we studied (25–330 K), all water clusters undergo significant structural changes. The trends are treelike structures dominating at high temperature and single-ring structures appearing in slightly lower temperatures. For n⩾7, an additional transition from single ring to multiple rings appears as the temperature decreases. Only for n⩾16 do polyhedral structures dominate ...

Journal ArticleDOI
TL;DR: The energy and properties of the zero-point state are focused on in the rigorous DMC calculations and variational calculations in full dimensionality for selected vibrational states of H(5)O(2) (+) using a new ab initio potential energy surface.
Abstract: We report quantum diffusion Monte Carlo (DMC) and variational calculations in full dimensionality for selected vibrational states of H(5)O(2) (+) using a new ab initio potential energy surface [X. Huang, B. Braams, and J. M. Bowman, J. Chem. Phys. 122, 044308 (2005)]. The energy and properties of the zero-point state are focused on in the rigorous DMC calculations. OH-stretch fundamentals are also calculated using "fixed-node" DMC calculations and variationally using two versions of the code MULTIMODE. These results are compared with infrared multiphoton dissociation measurements of Yeh et al. [L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 (1989)]. Some preliminary results for the energies of several modes of the shared hydrogen are also reported.

Journal ArticleDOI
TL;DR: A full nine-dimensional interaction potential for H2O-H2 calibrated using high-accuracy, explicitly correlated wave functions and all degrees of freedom are included using a systematic procedure transferable to other small molecules of astrophysical or atmospherical relevance.
Abstract: The hydrogen and water molecules are ubiquitous in the Universe. Their mutual collisions drive water masers and other line emission in various astronomical environments, notably molecular clouds and star-forming regions. We report here a full nine-dimensional interaction potential for H2O-H-2 calibrated using high-accuracy, explicitly correlated wave functions. All degrees of freedom are included using a systematic procedure transferable to other small molecules of astrophysical or atmospherical relevance. As a first application, we present rate constants for the vibrational relaxation of the upsilon(2) bending mode of H2O obtained from quasiclassical trajectory calculations in the temperature range of 500-4000 K. Our high-temperature (T >= 1500 K) results are found compatible with the single experimental value at 295 K. Our rates are also significantly larger than those currently used in the astrophysical literature and will lead to a thorough reinterpretation of vibrationally excited water emission spectra from space.

Journal ArticleDOI
TL;DR: A global potential energy surface has been constructed in order to determine the rate constants for atmospherically important reactions involving mercury and bromine and the implications of the obtained results to the description of the mechanism of recently observed polar tropospheric mercury depletion events are briefly discussed.
Abstract: A global potential energy surface (PES) for the 1A‘ ground state of HgBr2 has been constructed in order to determine the rate constants for atmospherically important reactions involving mercury and bromine. The total energy of HgBr2 was calculated by the multireference configuration interaction level of theory with series of correlation consistent basis sets up to quadruple-ζ quality with subsequent extrapolation to the complete basis set limit. An additive correction for spin−orbit coupling was also included. The global PES was represented piecewise by interpolating three separate parts of the surface with the reproducing kernel Hilbert space method and connecting them smoothly by switch functions. Quasiclassical trajectory calculations carried out on the surface yielded 298 K thermal rate constants of 3.89 × 10-11 cm3/(mol·s) for the abstraction reaction HgBr + Br → Hg + Br2, 2.98 × 10-11 cm3/(mol·s) for the recombination reaction Br + HgBr → HgBr2, and 3.97 × 10-11 cm3/(mol·s) for the exchange reaction...

Journal ArticleDOI
TL;DR: The potential energy surface for the reaction between OH and acetylene has been calculated using the RQCISD(T) method and extrapolated to the complete basis-set limit and, in contrast to previous models, ketene + H is found to be the main product at normal combustion conditions.
Abstract: The potential energy surface for the reaction between OH and acetylene has been calculated using the RQCISD(T) method and extrapolated to the complete basis-set limit. Rate coefficients were determined for a wide range of temperatures and pressures, based on this surface and the solution of the one-dimensional and two-dimensional master equations. With a small adjustment to the association energy barrier (1.1 kcal/mol), agreement with experiments is good, considering the discrepancies in such data. The rate coefficient for direct hydrogen abstraction is significantly smaller than that commonly used in combustion models. Also in contrast to previous models, ketene + H is found to be the main product at normal combustion conditions. At low temperatures and high pressures, stabilization of the C2H2OH adduct is the dominant process. Rate coefficient expressions for use in modeling are provided.

Journal ArticleDOI
TL;DR: Electron density distribution computed using the MP2(FC), B3LYP, and Hartree-Fock methods with the same basis set is studied and it is found that ED topology does not depend on the method of calculation.
Abstract: The potential energy surface for the benzene dimer in stacked conformations (84 points calculated) was computed at the MP2(FC)∕6-31+G(2d,2p) level of theory. Electron density (ED) distribution computed using the MP2(FC), B3LYP, and Hartree–Fock methods with the same basis set is studied in the frame of topological analysis. It is found that ED topology does not depend on the method of calculation. The values of the ED and its Laplacian in the cage critical point calculated using different methods are determined to be linearly dependent with the slope depending on basis set. Correlation equations based on these properties allow the interaction energy between benzene rings to be predicted with 8% mean relative error in the energy for the given region of the potential energy surface. This provides a new method for the estimation of stacking interaction energy using ED properties calculated with low level quantum-chemical methods.

Journal ArticleDOI
TL;DR: An accurate theoretical prediction of the vibrational spectra for a pure nitrogen ring (cyclic-N(3)) molecule is obtained up to the energy of the (2)A(2)/(2)B(1) conical intersection and an unusually large magnitude of the geometric phase effects is found.
Abstract: An accurate theoretical prediction of the vibrational spectra for a pure nitrogen ring (cyclic-N3) molecule is obtained up to the energy of the 2A2/2B1 conical intersection. A coupled-channel approach using the hyperspherical coordinates and the recently published ab initio potential energy surface [D. Babikov, P. Zhang, and K. Morokuma, J. Chem. Phys. 121, 6743 (2004)] is employed. Two independent sets of calculations are reported: In the first set, the standard Born–Oppenheimer approximation is used and the geometric phase effects are totally neglected. In the second set, the generalized Born–Oppenhimer approximation is used and the geometric phase effects due to the D3h conical intersection are accurately treated. All vibrational states are analyzed and assigned in terms of the normal vibration mode quantum numbers. The magnitude of the geometric phase effect is determined for each state. One important finding is an unusually large magnitude of the geometric phase effects in the cyclic-N3: it is ∼100 c...

Journal ArticleDOI
TL;DR: It is shown that minute variations of the interaction potential due to different fitting procedures may alter the Zeeman relaxation rate at ultralow temperatures by as much as 50%.
Abstract: A detailed analysis of the He‐ NHs 3 S ˛ d van der Waals complex is presented. We discuss ab initio calculations of the potential energy surface and fitting procedures with relevance to cold collisions, and we present accurate calculations of bound energy levels of the triatomic complex as well as collisional properties of NH molecules in a buffer gas of 3 He. The influence of the external magnetic field used to trap the NH molecules and the effect of the atom‐molecule interaction potential on the collisionally induced Zeeman relaxation are explored. It is shown that minute variations of the interaction potential due to different fitting procedures may alter the Zeeman relaxation rate at ultralow temperatures by as much as 50%. © 2005 American Institute of Physics . fDOI: 10.1063/1.1857473g

Journal ArticleDOI
TL;DR: The usefulness of the method is examined with dynamics in a two-dimensional system, showing that the one-dimensional surface obtained can predict the existence of an intermediate and the occurrence of path switching without a priori knowledge of the morphology of the original surface.
Abstract: In general, finding a one-dimensional representation of the kinetics of a high-dimensional system is a great simplification for the study of complex systems. Here, we propose a method to obtain a reaction coordinate whose potential of the mean force can reproduce the commitment probability distribution from the multidimensional surface. We prove that such a relevant one-dimensional representation can be readily calculated from the equilibrium distribution of commitment probabilities, which can be obtained with simulations. Also, it is shown that this representation is complementary to a previously proposed one-dimensional representation based on a quadratic approximation of the potential energy surface. The usefulness of the method is examined with dynamics in a two-dimensional system, showing that the one-dimensional surface thus obtained can predict the existence of an intermediate and the occurrence of path switching without a priori knowledge of the morphology of the original surface. The applicabilit...

Journal ArticleDOI
TL;DR: Minima on the seams of crossing between the doublet and quartet potential surfaces are found to lie substantially higher in energy than the cyclic N(3) minima, which strongly suggests thatcyclic N (3) possesses a long collision-free lifetime even if formed with substantial internal excitation.
Abstract: A comprehensive study of the unimolecular dissociation of the N3 radical on the ground doublet and excited quartet potential energy surfaces has been carried out with multireference single and double excitation configuration interaction and second-order multireference perturbation methods. Two forms of the N3 radical have been located in the linear and cyclic region of the lowest doublet potential energy surface with an isomerization barrier of 62.2 kcal/mol above the linear N3. Three equivalent C2v minima of cyclic N3 are connected by low barrier, meaning the molecule is free to undergo pseudorotation. The cyclic N3 is metastable with respect to ground state products, N(4S)+N2, and dissociation must occur via intersystem crossing to a quartet potential energy surface. Minima on the seams of crossing between the doublet and quartet potential surfaces are found to lie substantially higher in energy than the cyclic N3 minima. This strongly suggests that cyclic N3 possesses a long collision-free lifetime even if formed with substantial internal excitation.

Journal ArticleDOI
TL;DR: In this paper, the authors developed a new potential energy surface for spin-polarized K($2$S) + K$_{2}(^3\Sigma^+_u)$ collisions and carried out quantum dynamical calculations of vibrational quenching at low and ultralow collision energies for both bosons and fermions.
Abstract: We have developed a new potential energy surface for spin-polarized K($^2$S) + K$_{2}(^3\Sigma^+_u)$ collisions and carried out quantum dynamical calculations of vibrational quenching at low and ultralow collision energies for both bosons $^{39}$K and $^{41}$K and fermions $^{40}$K. At collision energies above about 0.1 mK the quenching rates are well described by a classical Langevin model, but at lower energies a fully quantal treatment is essential. We find that for the low initial vibrational state considered here ($v=1$), the ultracold quenching rates are {\it not} substantially suppressed for fermionic atoms. For both bosons and fermions, vibrational quenching is much faster than elastic scattering in the ultralow-temperature regime. This contrasts with the situation found experimentally for molecules formed via Feshbach resonances in very high vibrational states.


Journal ArticleDOI
TL;DR: This work presents a perspective on the computation and interpretation of force constants at points of symmetry-induced (Jahn-Teller) conical intersection, based upon the projection of the 'branching space' from the full (3N - 6)-dimensional Hessian for each component of a degenerate electronic state.
Abstract: We present a perspective on the computation and interpretation of force constants at points of symmetry-induced (Jahn-Teller) conical intersection. Our method is based upon the projection of the 'branching space' from the full (3N - 6)-dimensional Hessian for each component of a degenerate electronic state. For Jahn-Teller active molecules, this has the effect of removing the linear Jahn-Teller coupling from all but the interstate coupling and gradient difference vectors. The quadratic coupling constants are determined by the splitting of the harmonic vibrational frequencies within degenerate vibrational normal coordinates of the 'intersection space'. The potential energy surface topology along these normal modes is analogous to the Renner-Teller effect occurring in orbitally degenerate linear molecules. Our methodology gives a straightforward theoretical analysis of the various Jahn-Teller intersections and allows the determination of the seam curvature. Thus, we are in a position to compute the various Jahn-Teller coupling constants, in a particular coordinate system, and in addition to determine the nature of the high-symmetry Jahn-Teller geometry (i.e., minimum or saddle-point on the seam). We illustrate these concepts with various examples of different Jahn-Teller conical intersections in some small molecules.

Journal ArticleDOI
01 Jan 2005
TL;DR: In this paper, the reaction between O2 and the armchair surface of a model graphite molecule has been studied using density functional calculations at the B3LYP/6-31G(d) level of theory.
Abstract: The reaction between O2 and the armchair surface of a model graphite molecule has been studied using density functional calculations at the B3LYP/6-31G(d) level of theory. Both equilibrium and transition state geometries were optimized to provide a fundamental understanding of the energetics and kinetics of the chemisorption, desorption, rearrangement, and migration reactions that contribute to carbon gasification. A small barrier of 18 kJ mol−1 was found for the chemisorption reaction, which is 578 kJ mol−1 exothermic overall, producing a stable quinone. A number of reaction pathways with barriers below 578 kJ mol−1 were characterized. Gasification of carbon occurs as CO, with barriers of 296 and 435 kJ mol−1 for the first and second CO loss, respectively. The stable quinone can also undergo a rearrangement reaction to form two ketene groups, with a barrier of 260 kJ mol−1. If the armchair edge is extended to include an adjacent aromatic ring, the oxide can migrate along the surface. This initially forms a furan-like bridge structure, with a barrier of just 89 kJ mol−1. A further barrier of 383 kJ mol−1 leads to CO desorption from the furan. The furan can also rearrange further with a barrier of 212 kJ mol−1 to form a five-membered lactone, the most stable structure identified on the potential energy surface. Rearrangement and migration reactions, which have not generally been incorporated into carbon gasification models, are shown to be potentially important pathways in carbon oxidation reactions.

Journal ArticleDOI
TL;DR: A state‐of‐the‐art benchmark potential energy surface (PES) is computed for the archetypal oxidative addition of the ethane CC bond to the palladium atom and the performance of 24 popular density functionals, covering LDA, GGA, meta‐GGA, and hybrid density functional, for describing this reaction is evaluated.
Abstract: We have computed a state-of-the-art benchmark potential energy surface (PES) for the archetypal oxidative addition of the ethane C-C bond to the palladium atom and have used this to evaluate the performance of 24 popular density functionals, covering LDA, GGA, meta-GGA, and hybrid density functionals, for describing this reaction. The ab initio benchmark is obtained by exploring the PES using a hierarchical series of ab initio methods [HF, MP2, CCSD, CCSD(T)] in combination with a hierarchical series of five Gaussian-type basis sets, up to g polarization. Relativistic effects are taken into account either through a relativistic effective core potential for palladium or through a full four-component all-electron approach. Our best estimate of kinetic and thermodynamic parameters is -10.8 (-11.3) kcal/mol for the formation of the reactant complex, 19.4 (17.1) kcal/mol for the activation energy relative to the separate reactants, and -4.5 (-6.8) kcal/mol for the reaction energy (zero-point vibrational energy-corrected values in parentheses). Our work highlights the importance of sufficient higher angular momentum polarization functions for correctly describing metal-d-electron correlation. Best overall agreement with our ab initio benchmark is obtained by functionals from all three categories, GGA, meta-GGA, and hybrid DFT, with mean absolute errors of 1.5 to 2.5 kcal/mol and errors in activation energies ranging from -0.2 to -3.2 kcal/mol. Interestingly, the well-known BLYP functional compares very reasonably with a slight underestimation of the overall barrier by -0.9 kcal/mol. For comparison, with B3LYP we arrive at an overestimation of the overall barrier by 5.8 kcal/mol. On the other hand, B3LYP performs excellently for the central barrier (i.e., relative to the reactant complex) which it underestimates by only -0.1 kcal/mol.