Showing papers on "Potential energy surface published in 1992"

A novel discrete variable representation for quantum mechanical reactive scattering via the S-matrix Kohn method

Abstract: A novel discrete variable representation (DVR) is introduced for use as the L2 basis of the S‐matrix version of the Kohn variational method [Zhang, Chu, and Miller, J. Chem. Phys. 88, 6233 (1988)] for quantum reactive scattering. (It can also be readily used for quantum eigenvalue problems.) The primary novel feature is that this DVR gives an extremely simple kinetic energy matrix (the potential energy matrix is diagonal, as in all DVRs) which is in a sense ‘‘universal,’’ i.e., independent of any explicit reference to an underlying set of basis functions; it can, in fact, be derived as an infinite limit using different basis functions. An energy truncation procedure allows the DVR grid points to be adapted naturally to the shape of any given potential energy surface. Application to the benchmark collinear H+H2→H2+H reaction shows that convergence in the reaction probabilities is achieved with only about 15% more DVR grid points than the number of conventional basis functions used in previous S‐matrix Kohn...

Cooperative (nonpairwise) effects in water trimers: An ab initio molecular orbital study

Abstract: High levels of ab initio molecular orbital theory were used to study the structures and binding energies of water trimers. These calculations included HF/6‐311++G(2df,2p) geometry optimizations for the 17 structures considered. Harmonic vibrational energies were obtained at the HF/6–311++G(2d,2p) level. The HF potential energy surface present three minima whose geometries were refined at the MP2/6–311+G(d,p) level. The global minimum corresponds to an asymmetric cyclic structure which presents significant cooperative effects with respect to the Cs dimer. To properly describe these nonpairwise effects, ZPE (zero point energy) and correlation corrections must be taken into account. They are reflected in a stiffer intermolecular potential, shorter O–O distances, longer donor O–H bond lengths, larger energies per hydrogen bond (HB), and greater shifts of the donor O–H bond stretching frequencies than the Cs dimer. Contrarily, the other two local minima present HBs which are weaker than those of the dimer. The...

Vibrational Modes and the Dynamic Solvent Effect in Electron and Proton Transfer

Abstract: This article primarily reviews recent work on ultrafast experiments on excited state intramolecular electron and proton transfer, with an emphasis on experiments on chemical systems that have been analyzed theoretically. In particular, those systems that have been quantitatively characterized by static spectroscopy, which provides detailed information about the reaction potential energy surface and about other parameters that are necessary to make a direct comparison to theoretical predictions, are described.

Photodissociation of water in the first absorption band: A prototype for dissociation on a repulsive potential energy surface

Abstract: The photodissociation of water in the first absorption band, H{sub 2}O(X) + {Dirac_h}{omega} {yields} H{sub 2}O(A{sup 1}B{sub 1}) {yields} H({sup 2}S) + OH({sup 2}II), is a prototype of fast and direct bond rupture in an excited electronic state. It has been investigated from several perspectives-absorption spectrum, final state distributions of the products, dissociation of vibrationally excited states, isotope effects, and emission spectroscopy. The availability of a calculated potential energy surface for the A state, including all three internal degrees of freedom, allows comparison of all experimental data with the results of rigorous quantum mechanical calculations without any fitting parameters or simplifying model assumptions. As the result of the confluence of ab initio electronic structure theory, dynamical theory, and experiment, water is probably the best studied and best understood polyatomic photodissociation system. In this article we review the joint experimental and theoretical advances which make water a unique system for studying molecular dynamics in excited electronic states. We focus our attention especially on the interrelation between the various perspectives and the correlation with the characteristic features of the upper-state potential energy surface. 80 refs., 14 figs.

Quasiclassical state to state reaction cross sections for D+H2(v=0, j=0)→HD(v’,j’)+H. Formation and characteristics of short‐lived collision complexes

Abstract: State resolved total and differential reaction cross sections, as well as reaction probabilities, have been calculated by the quasiclassical trajectory (QCT) method for the D+H2(v=0, j=0)→HD(v’,j’)+H reaction on the Liu–Siegbahn–Truhlar–Horowitz potential energy surface in the collision energy range 0.30–1.25 eV. Thus a detailed comparison with existing fully converged quantum mechanical (QM) calculations has been performed. The general agreement between both sets of results is good with some differences. QCT integral reaction cross sections for the production of HD(v’=0) are lower than the corresponding QM ones by 10%–15% for collision energies higher than 0.6 eV, and the energy dependence of the QCT reaction probability with a total angular momentum J equal to zero shows no structure when summed over all j’ states (contrary to the QM case). The differential cross sections for the lowest j’ values show, when represented as a function of energy, a ‘‘ridge’’ feature similar to the one found in exact QM cal...

Quasiclassical trajectory calculations of the thermal rate coefficients for the reactions H(D)+O2→OH(D)+O and O+OH(D)→O2+H(D) as a function of temperature

Abstract: Thermal rate coefficients are calculated for the reaction, (1), H+O2→OH+O and its reverse, (−1), O+OH→O2+H, using the quasiclassical trajectory method and the most recently reported double many‐body expansion (DMBE IV) potential energy surface for the ground electronic state of the hydroperoxyl radical. The full range of temperatures for which experimental data is available in the literature has been covered, namely, 1000≤T≤3000 K for reaction (1) and 150≤T≤3000 K for reaction (−1). The equilibrium constant has also been evaluated. In addition, calculations of the isotopic effect on the thermal rate coefficient for the deuterated reactions D+O2→OD+O and O+OD→O2+D, and equilibrium constant, are reported. The theoretical results are shown to be in good agreement with the most recent and accurate experimental measurements.

State-to-State Rates for the D + H2(v = 1, j = 1) → HD(v', j') + H Reaction: Predictions and Measurements

Abstract: A fully quantal wavepacket approach to reactive scattering in which the best available H3 potential energy surface was used enabled a comparison with experimentally determined rates for the D + H2(v = 1, j = 1) → HD(v9 = 0, 1, 2; j9) + H reaction at significantly higher total energies (1.4 to 2.25 electron volts) than previously possible. The theoretical results are obtained over a sufficient range of conditions that a detailed simulation of the experiment was possible, thus making this a definitive comparison of experiment and theory. Good to excellent agreement is found for the vibrational branching ratios and for the rotational distributions within each product vibrational level. However, the calculated rotational distributions are slightly hotter than the experimentally measured ones. This small discrepancy is more marked for products for which a larger fraction of the total energy appears in translation. The most likely explanation for this behavior is that refinements are needed in the potential energy surface.

Highly excited vibrational eigenstates of nonlinear triatomic molecules. Application to H2O

Abstract: Highly accurate quantum‐mechanical calculations are presented for highly excited vibrational states of H2O. The vibration Hamiltonian operator Hvib for a nonlinear triatomic molecule is given in Radau coordinates. A direct product basis is chosen, and the Hvib matrix is evaluated in the discrete variable representation (DVR) for the symmetrized Radau coordinates. Vibrational eigenstates are computed from the DVR Hvib via the successive diagonalization/truncation technique. A comparison of the computed eigenvalues with those observed demonstrate the accuracy of our model. Highly excited vibrational states, up to 30 000 cm−1 above the zero‐point energy, are reported for the potential energy surface (PES) given by Jensen [J. Mol. Phys., 133, 438 (1989)]. Using natural orbital expansions, the eigenfunctions of vibrational states are analyzed to understand the origins of the dynamical mixing of the vibrational modes. The local/normal mode transitions, Fermi resonances, Darling–Dennison interactions, and the mode separabilities are investigated. Statistical studies on the energy level spacings are presented for two different types of PES.

Ab initio molecular dynamics with correlated molecular wave functions: Generalized valence bond molecular dynamics and simulated annealing

Abstract: We present an ab initio molecular dynamics algorithm at the generalized valence bond level. It does not need a precalculated potential energy surface or model Hamiltonian; instead the nuclei move according to first principles forces derived from the electronic wave function which in turn follows the movement of the nuclei. This technique includes the dominant static electron correlations, it can treat ground and excited many‐electron states, and it can describe chemical bond formation and breaking qualitatively correctly. We apply the method to Na4, as a generic test example for small metal clusters, and show spin‐dependent free dissociation dynamics as well as geometry optimization by simulated annealing. The latter involves novel boundary conditions to prevent dissociation and mass scaling to enhance performance.

Measurement of the ν8 intermolecular vibration of (D2O)2 by tunable far infrared laser spectroscopy

Abstract: The first accurate measurement of an intermolecular vibration of the water dimer is reported. Five vibration–rotation‐tunneling (VRT) bands of the perdeuterated isotope, located near 84 cm−1, have been assigned to the A1/E/B1 tunneling components of the Ka=0←0 and Ka=1←0 subbands. The vibration involves large amplitude motion of the hydrogen bond acceptor and is assigned as the ν8 acceptor wag. The spectra indicate strong coupling of both the donor–acceptor interconversion and donor tunneling motions to the excited vibrational coordinate. This measurement provides a benchmark for future efforts toward the determination of an accurate potential energy surface for the water dimer.

Hydrogen-bond swapping in the benzene-water complex. A model study of the interaction potential

Abstract: Model calculations have been carried out to characterize the detailed nature of the benzene-water intermolecular interaction potential energy surface. At equilibrium, the calculations show a binding energy of 1303 cm -1 . The optimum water molecule's position is above the plane of the benzene ring and shifted from benzene's symmetry axis by 0.82 A. Of course, the equilibrium position alone does not satisfactorily characterize the benzene-water interactions because of the extreme floppiness of the complex

Femtosecond vibrational transition‐state dynamics in a chemical reaction

Abstract: We report the femtosecond dynamics of the vibrational motion in the transition-state evolution of an isomerization reaction. The observed nonstatistical and bound behavior reflects the localization in selective torsional and bending modes (of the 72 normal modes of the system). The multidimensionality of the potential energy surface is examined by comparing experiments with theoretical calculations.

Accurate potential energy surface for Xe/Pt(111): A benchmark gas/surface interaction potential

Abstract: We have determined an empirical potential energy function for the interaction of xenon with the Pt(111) surface which is consistent with a wide range of dynamical and equilibrium experimental data. These include scattering measurements, with detailed angular distributions and energy transfer data, at incidence energies from 0.5 to 14.3 eV. Also used are thermal desorption rates and trapping probabilities, as well as thermodynamic properties of monolayer phases including the ‘‘energy jump’’ at the transition from the commensurate to the uniaxially compressed incommensurate phase. The potential also agrees with an experimental value for the frequency of vibration normal to the surface, and has the correct asymptotic behavior at large distances from the surface (V=−c3/z3, with an experimental estimate of c3). The equilibrium position for a single Xe atom lies directly above a surface platinum atom, and the calculated height above this atom is 3.35 A.

Chaos in small clusters of inert gas atoms

Abstract: We have calculated the Kolmogorov entropy for three‐ and seven‐particle clusters bound by Lennard‐Jones potentials and for three‐particle clusters bound by Morse potentials of various ranges. We have used two quite different methods, one of which is new, which give consistent results. We find that all of these systems are classically chaotic over a wide range of energies surrounding the estimated quantum‐mechanical zero point energies. Furthermore, for the three‐particle clusters, we can rationalize the variation in the degree of chaos with total energy in terms of the local structure of the clusters’ potential energy surface.

First principles computation of thermochemical properties beyond the harmonic approximation. I. Method and application to the water molecule and its isotopomers

Abstract: The anharmonic potential energy surface of water has been computed ab initio using an augmented coupled cluster method and various basis sets. Whereas the Pople 6–311 G family is manifestly unsatisfactory, Huzinaga–Dunning basis sets perform quite well. The [5s4p2d1f,3s2p] surface reproduces harmonic frequencies and anharmonicity constants to better than about 2 and 1 cm−1, respectively. For quantitative agreement with experiment, both f functions on oxygen and inclusion of core correlation seem to be prerequisite. Comparison with various experimentally derived force fields reveals that the ab initio force field is of comparable quality. From the best computed force field, a set of spectroscopic constants has been derived for all important isotopomers of water. Using a hybrid analytic/direct summation method recently developed by the present authors, the thermodynamic functions gef(T), hcf(T), S0, and Cp are computed including exact account of anharmonicity and rovibrational coupling, and very good analyt...

Dissociation path for H2 on Al(110).

Abstract: The minimum energy path is calculated for an H 2 molecule dissociating on an Al(110) surface within local density functional theory. The properties of the potential energy surface along the five H 2 ionic coordinates perpendicular to the reaction path are also determined and shown to be essential for an understanding of the dissociation dynamics

Aspects of the reaction mechanism of ethane combustion. Conformations of the ethylperoxy radical

Abstract: The detailed molecular understanding of hydrocarbon combustion processes is recognized as an important goal. The specific system studied-via ab initio theoretical methods - in this work is the reaction between the ethyl radical (C{sub 2}H{sub 5}{degrees}) and molecular oxygen (O{sub 2}). It may be argued that further experimental work on this system will yield limited new insights until theoretical investigations of the potential energy surface are made. Theoretical methods used include self-consistent-field (SCF) and configuration interaction including all single and double excitations (CISD) with up to double zeta plus polarization (DZP) quality basis sets. A total of 55 distinct stationary points on the C{sub 2}H{sub 5}(O82){degrees} potential energy surface were considered here. Two excited states of the ethylperoxy radical, six conformers of the ground {sup 2}A inch state, and four conformers of the excited {sup 2}A (foot), state were studied. For the ground-state surface, the barrier between the staggered and gauche structures is only 1.0 kcal mol{sup {minus}1}. The barrier to internal rotation of the methyl group is between 2.5 and 3.0 kcal mol{sup {minus}1} depending on the level of theory. 74 refs., 3 figs., 7 tabs.

Application of the diffusion equation method of global optimization to water clusters

Abstract: In this paper, we investigate the behavior of the recently developed diffusion equation method (DEM) for water cluster The DEM is a global optimization method that involves the deformation of the potential energy surface to remove local minima of high energy. The deformed potential energy surface corresponds to the solution F(x,t) of the diffusion original energy function f(x) as the initial condition, i.e., F(x,0)=f(x)

Vibrational predissociation of the Ar⋅⋅⋅Cl2 van der Waals complex: The small molecule limit for intramolecular vibrational redistribution

Abstract: A converged three‐dimensional quantum treatment of vibrational predissociation in the Ar⋅⋅⋅Cl2(B3Π0u+,υ′) van der Waals complex is presented. The potential energy surface used is a sum of pairwise Morse atom–atom interactions adjusted asymptotically to a C6/R6+C8/R8 anisotropic van der Waals form. Calculations have been performed in the energy region of Ar⋅⋅⋅Cl2(B,υ’=6, 10, and 11) excited levels. In agreement with the experimental findings, the final rotational distribution of Cl2 is found to be strongly dependent on the initial υ’ state being excited, as well as on the number of vibrational quanta lost in the vibrational predissociation process. The role of intramolecular vibrational redistribution for υ’=10 and 11 for which the Δυ=−1 channel is closed is also studied. It is found that the vibrational predissociation (VP) dynamics are dominated by the coupling of the zero‐order ‘‘bright’’ state with a single ‘‘dark’’ state from the υ’−1 manifold of van der Waals vibrationally excited states which then d...

Closs's diradical: some surprises on the potential energy hypersurface

Abstract: The singlet and triplet potential energy surfaces for 1,3-cyclopentanediyl (Closs's diradical) have been investigated using ab initio electronic structure theory. The triplet C 2υ structure previously postulated to be an intermediate in the ring inversion of bicyclo[2.1.0]pentane (BCP) is found to correspond to a saddle point, rather than a minimum, on a potential energy surface more complex than that originally proposed by Closs. The singlet and triplet surfaces share several qualitative features, but the triplet stationary points lie be ∼1 kcal/mol below the corresponding singlets

A quasiclassical trajectory study of OH rotational excitation in OH+CO collisions using ab initio potential surfaces

Abstract: We have performed large basis set configuration interaction calculations to characterize the two potential surfaces ({sup 2}{ital A}{prime} and {sup 2}{ital A}{double prime} ) which correlate to the ground state of OH+CO. Only planar geometries of the four atoms are considered, and the calculations restrict the OH and CO bond distances to their isolated diatomic values. Global representations of these potential surfaces have been developed and used in quasiclassical trajectory studies of rotational excitation in low energy (1--6 kcal/mol) collisions of OH and CO in their respective rovibrational ground states. We find that the collisional excitation cross sections are about equal for the two surfaces, and there is a monotonic increase in each cross section with translational energy. For OH rotational quantum numbers {ital N} between 2 and 6 there is approximately a factor of 2--3 decrease in the cross section for each unit increase in {ital N}. The energy and {ital N} dependence of these cross sections are generally in excellent agreement with recent experiments. We have also explored the sensitivity of these cross sections to the nature of the potential energy surface, and we have used a surface that describes the formation of the intermediate complex HOCO tomore » determine sensitivity of the rotationally inelastic cross sections to complex formation. In agreement with the experiments, we find that the low energy, high {ital N} cross sections are appreciably perturbed by complex formation.« less

Dynamics of the N(4Su) + NO(X 2Π) → N2(X 1Σg+) + O(3Pg) atmospheric reaction on the 3A‘ ground potential energy surface. I. Analytical potential energy surface and preliminary quasiclassical trajectory calculations

Abstract: The N(4Su)+NO(X 2Π)→N2(X 1Σg+)+O(3Pg) reaction plays an important role in the upper atmosphere chemistry and as a calibration system for discharge flow systems. Surprisingly, very little theoretical and experimental work has been devoted to the characterization of the dynamical features of this system. In this work a Sorbie–Murrell expression for the lowest 3A‘ potential energy surface (PES) connecting reactants in their ground electronic states based upon the fitting of an accurate ab initio CI grid of points has been derived. The PES fitted shows no barrier to reaction with respect to the reactants asymptote in accordance with experimental findings and becomes highly repulsive as the NNO angle is varied away from the saddle point geometry. The results of preliminary quasiclassical trajectory calculations on this surface reproduce very well the experimental energy disposal in products, even though the vibrational distribution derived from trajectories seems to be a bit cooler than the experimental data. ...

A method and results for realistic molecular dynamic simulation of hydrogenated amorphous carbon structures using a scheme consisting of a linear combination of atomic orbitals with the local-density approximation

Abstract: A method for realistic molecular dynamic (MD) simulations of the chemical bonding formation in extended hydrogenated amorphous carbon (a-C:H) structures of varying density and incorporated hydrogen content is presented. Applying the Born-Oppenheimer approximation, the forces moving the atoms via MD on the potential energy surface are calculated within an approximated MD-density functional theory which uses localized basis functions. The method is shown to describe correctly the ground state configurations of Cn microclusters, CnHm hydrocarbon molecules and radicals, as well as bulk crystalline carbon. Application to dynamical structure simulation of a-C and a-C:H results in realistic metastable configurations which are characterized electronically by a well defined gap in the electronic density of states around the Fermi energy. A reasonable structure statistics is obtained and compared with fully ab initio calculations and experiments.