Showing papers on "Potential energy surface published in 1998"

Lowest Energy Structures of Gold Nanoclusters

Abstract: The lowest energy structures of ${\mathrm{Au}}_{n}$ ( $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}38,55,75$) nanoclusters are obtained by unconstrained dynamical and genetic-symbiotic optimization methods, using a Gupta $n$-body potential. A set of amorphous structures, nearly degenerate in energy, are found as the most stable configurations. Some crystalline or quasicrystalline isomers are also minima of the cluster potential energy surface with similar energy. First principles calculations using density functional theory confirm these results and give different electronic properties for the ordered and disordered gold cluster isomers.

On the chemical mechanism of surface enhanced Raman scattering: Experiment and theory

Abstract: We have investigated the chemical mechanism of surface enhanced Raman scattering (SERS) on an atomically smooth metal surface using electron energy loss spectroscopy (EELS) and molecular spectroscopy simulations. The EEL spectra of pyromellitic dianhydride (PMDA) adsorbed on Cu(100) and Cu(111) are reported. Simulations of the surface-enhanced Raman spectra and electron energy loss spectra (EELS) of pyromellitic dianhydride adsorbed on Cu(100) and Cu(111) are reported. The surface enhanced Raman spectra [J. Chem. Soc. Faraday Trans. 92, 4775 (1996)] and the EEL spectra are shown to be sensitive to crystal face. The relevant excited state observed in the EEL spectrum is not intrinsic to molecular PMDA, but results from chemisorption. The Raman spectra are sensitive to the incident laser polarization on both the (100) and (111) surfaces but in different ways. These observations are shown to be a result of the excited state potential energy surface having different shape, and the respective transition dipole...

Global Optimization by Basin-Hopping and the Lowest Energy Structures of Lennard-Jones Clusters Containing up to 110 Atoms

Abstract: We describe a global optimization technique using `basin-hopping' in which the potential energy surface is transformed into a collection of interpenetrating staircases. This method has been designed to exploit the features which recent work suggests must be present in an energy landscape for efficient relaxation to the global minimum. The transformation associates any point in configuration space with the local minimum obtained by a geometry optimization started from that point, effectively removing transition state regions from the problem. However, unlike other methods based upon hypersurface deformation, this transformation does not change the global minimum. The lowest known structures are located for all Lennard-Jones clusters up to 110 atoms, including a number that have never been found before in unbiased searches.

Chemical reaction dynamics beyond the born-oppenheimer approximation

Abstract: ▪ Abstract To predict the branching between energetically allowed product channels, chemists often rely on statistical transition state theories or exact quantum scattering calculations on a single adiabatic potential energy surface. The potential energy surface gives the energetic barriers to each chemical reaction and allows prediction of the reaction rates. Yet, chemical reactions evolve on a single potential energy surface only if, in simple terms, the electronic wavefunction can evolve from the reactant electronic configuration to the product electronic configuration on a time scale that is fast compared to the nuclear dynamics through the transition state. The experiments reviewed here investigate how the breakdown of the Born-Oppenheimer approximation at a barrier along an adiabatic reaction coordinate can alter the dynamics of and the expected branching between molecular dissociation pathways. The work reviewed focuses on three questions that have come to the forefront with recent theory and exper...

Polyatomic molecular potential energy surfaces by interpolation in local internal coordinates

Abstract: We present a method for expressing a potential energy surface (PES) for polyatomic molecules as an interpolation of local Taylor expansions in internal coordinates. This approach extends and replaces an earlier method which was only directly applicable to molecules of no more than four atoms. In general, the local Taylor expansions are derived from ab initio quantum calculations. Here, the methodology is evaluated by comparison with an analytic surface for the reactions H+CH4⇌H2+CH3. Approximately 1000–1300 data points are required for an accurate 12-dimensional surface which describes both forward and backward reactions, at the energy studied.

Sub-microhartree accuracy potential energy surface for H3+ including adiabatic and relativistic effects. I. Calculation of the potential points

Abstract: Sixty-nine points of the Born–Oppenheimer (BO) potential energy surface (PES) for the ground state of H3+ have been computed using explicitly correlated Gaussian wave functions with optimized nonlinear parameters. The calculated points have an absolute error of about 0.02 cm−1 (0.1 microhartree), i.e., they are by at least one order of magnitude more accurate than ever reported. Similarly accurate adiabatic and relativistic corrections have also been evaluated by means of the Born–Handy formula and by direct perturbation theory (DPT), respectively.

Searching for saddle points of potential energy surfaces by following a reduced gradient

Abstract: The old coordinate driving procedure to find transition structures . in chemical systems is revisited. The well-known gradient criterion, =E x s 0, . which defines the stationary points of the potential energy surface PES , is reduced by one equation corresponding to one search direction. In this manner, abstract curves can be defined connecting stationary points of the PES. Starting at a given minimum, one follows a well-selected coordinate to reach the saddle of interest. Usually, but not necessarily, this coordinate will be related to the . reaction progress. The method, called reduced gradient following RGF , locally has an explicit analytical definition. We present a predictor)corrector method for tracing such curves. RGF uses the gradient and the Hessian matrix or updates of the latter at every curve point. For the purpose of testing a whole surface, the six-dimensional PES of formaldehyde, H CO, was explored by RGF 2 . using the restricted Hartree)Fock RHF method and the STO-3G basis set. Forty-nine minima and saddle points of different indices were found. At least seven stationary points representing bonded structures were detected in addition to those located using another search algorithm on the same level of theory. Further examples are the localization of the saddle for the HCN | CNH . isomerization used for steplength tests and for the ring closure of azidoazo- methine to 1 H-tetrazole. The results show that following the reduced gradient may represent a serious alternative to other methods used to locate saddle points in quantum chemistry. Q 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1087)1100, 1998

“Direct” and “Correct” Calculation of Canonical and Microcanonical Rate Constants for Chemical Reactions

Abstract: Theoretical approaches for calculating rate constants of chemical reactionseither the microcanonical rate for a given total energy k(E) or the canonical rate for a given temperature k(T)are described that are both “direct”, i.e., bypass the necessity of having to solve the complete state-to-state quantum reactive scattering problem, yet also “correct”, i.e., in principle exact (given a potential energy surface, assuming nonrelativistic quantum mechanics, etc.) Applications to a variety of reactions are presented to illustrate the methodology for various dynamical situations, e.g., transition-state-theory-like dynamics where the system moves directly through the interaction (transition-state) region and reactions that form long-lived collision complexes. It is also shown how this rigorous quantum theory can be combined with the Lindemann mechanism for describing the effects of collisions with a bath gas, so as to be able to treat recombination reactions and other effects of pressure. Finally, several ways ...

Revised Anisotropic Site Potentials for the Water Dimer and Calculated Properties

Abstract: Two new parametrizations of a recent ab initio polarizable anisotropic site potential for water are presented. The new versions improve the description of the electrostatic interactions, add an explicit charge-transfer term, and use more accurate dispersion coefficients from the recent literature. To assess the merits of the new models, the potential energy surface of the dimer is analyzed and a comparison is made with 12 other polarizable potentials for water in the literature, most of them being currently used in computer simulation. The structure, energy, and harmonic intermolecular frequencies of the stationary points have been determined and compared with the best available ab initio calculations. The energy barriers and pathways for hydrogen atom interchange within the dimer are discussed. The second virial coefficient B(T) of steam between 373 and 973 K, including first-order quantum corrections, is reported. For all the models, the quantum corrections are found to be significant at the lowest temp...

Femtosecond Dynamics of a Hydrogen-Bonded Model Base Pair in the Condensed Phase: Double Proton Transfer in 7-Azaindole

Abstract: Femtosecond, real-time dynamics of a hydrogen-bonded model base pair (7-azaindole dimer) in the condensed phase is presented and compared with gas-phase, molecular beam studies. Following the preparation of a wave packet (nonequilibrium state), we resolved the initial decay of the reactive pair and the rise of the tautomer in two solvents. Studies of the isotope effect, solvent viscosity, energy dependence, and polarization anisotropy are consistent with direct and indirect reaction pathways for the double proton transfer on a global potential energy surface.

High level ab initio and density functional theory studies on methanol–water dimers and cyclic methanol(water)2 trimer

Abstract: The methanol-water dimers and the potential energy surface of the cyclic methanol(water)2 trimer have been studied through the use of high-level ab initio calculations and density functional methods. The geometries have been optimized at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The harmonic vibrational frequencies were obtained at the latter level. The final energies of the different local minima were calculated in the framework of the G2 and G2(MP2) theories. These values were compared with those obtained using the B3LYP/6-311+G(3df,2p) approach. At all the levels of theory considered the most stable conformer of methanol-water heterodimers corresponds to that in which water behaves as a hydrogen bond donor, in agreement with the most recent experimental evidences [P. A. Stockman et al., J. Chem. Phys. 107, 3782 (1997)]. The energy differences between the different conformers of the cyclic methanol(water)2 trimer are rather small, as well as the energy barriers connecting them. The g...

Uracil Dimer: Potential Energy and Free Energy Surfaces. Ab Initio beyond Hartree−Fock and Empirical Potential Studies

Abstract: The first complete theoretical analysis of the gas-phase formation of a nucleic acid base pair (uracil dimer) has been performed. The study is based on a combination of AMBER 4.1 empirical potential, correlated ab initio quantum chemical methods, computer simulations, and statistical thermodynamical methods. In total, 11 low-energy minima structures were located on the potential energy surface of the uracil dimer: seven of them are H-bonded, one is T-shaped, and three correspond to various stacked arrangements. The most stable structure is a H-bonded dimer with two N1−H···O2 H-bonds, designated as HB4; it has an energy minimum of −15.9 kcal/mol at the MP2/6-31G*(0.25)//HF/6-31G** level of theory. T-shaped structure and stacked structures are less stable than H-bonded ones. Thermodynamic characteristics were obtained using the rigid rotor−harmonic oscillator−ideal gas (RR-HO-IG) approximation adopting the AMBER 4.1 and ab initio characteristics. Furthermore, the population of various structures was determ...

An accurate ab initio HOCl potential energy surface, vibrational and rotational calculations, and comparison with experiment

Abstract: Accurate ab initio multireference configuration interaction (CI) calculations with large correlation-consistent basis sets are performed for HOCl. After extrapolation to the complete basis set limit, the ab initio data are precisely fit to give a semiglobal three-dimensional potential energy surface to describe HOCl→Cl+OH from high overtone excitation of the OH-stretch. The average absolute deviation between the ab initio and fitted energies is 4.2 cm−1 for energies up to 60 kcal/mol relative to the HOCl minimum. Vibrational energies of HOCl including the six overtones of the OH-stretch are computed using a vibrational-Cl method on the fitted potential and also on a slightly adjusted potential. Near-spectroscopic accuracy is obtained using the adjusted potential; the average absolute deviation between theory and experiment for 19 experimentally reported states is 4.8 cm−1. Very good agreement with experiment is also obtained for numerous rotational energies for the ground vibrational state, the ClO-stretc...

The potential energy surface of guanine is not flat : an ab initio study with large basis sets and higher order electron correlation contributions

Abstract: An investigation of the potential energy surface (PES) of guanine was performed. The (nonplanar) molecular geometries of seven different forms of guanine were studied using the ab initio LCAO-MO method at the MP2 level with valence double-ζ basis sets augmented by d- and p-polarization functions (6-31G(d,p)). Among the studied forms are the four lowest energy tautomers, one rotamer, and two transition state forms for proton transfer between oxo and hydroxo tautomers of guanine N(7)H and N(9)H tautomers. Our best estimation of the relative stabilities of the tautomers includes electron correlation contributions calculated at the MP2/6-31G(d,p) reference geometries at the MP4(SDTQ)/6-31G(d,p), MP4(SDQ)/6-311G(d,p), and MP2/6-311++G(df,pd) levels. Three tautomers (and one rotamer) are located within 10 kJ mol-1 on the MP2 level PES of guanine. The calculated energy barriers for the proton transfer are larger than 150 kJ mol-1 and do not change by more than a few kJ mol-1 upon application of the SCRF model ba...

An ab initio path integral molecular dynamics study of double proton transfer in the formic acid dimer

Abstract: Double proton transfer in the formic acid dimer has been investigated with Car–Parrinello ab initio molecular dynamics calculations. The electronic structure of the dimer has been obtained using gradient-corrected density functional theory based on the B-LYP (Becke exchange [Phys. Rev. A 38, 3098 (1988)] and Lee–Yang–Parr correlation [Phys. Rev. B 37, 785 (1988)] functional. The optimized equilibrium and saddle-point geometries, obtained by simulated annealing, are in good agreement with previous ab initio quantum chemical predictions and experiment. Thermal and quantum fluctuations of nuclei along the double proton transfer reaction path have also been investigated at T=300 K. Thermal fluctuations give a broad distribution of nuclei around the minimum energy path on the potential energy surface. Quantum fluctuations, investigated using ab initio path integral molecular dynamics, make the distribution even broader around the equilibrium structure, and cause the distribution to deviate appreciably from the...

Quantum reactive scattering of O(1D)+H2 and O(1D)+HD

Abstract: A recently developed wavepacket method is applied to study the quantum dynamics of the reactions O(1D)+H2→OH+H and O(1D) +HD→OH(OD)+D(H) The abinitio based, global, 1A′ potential energy surface of Ho etal is employed The results of three-dimensional, total angular momentum J=0 scattering calculations are presented and discussed, with emphasis on qualitative features of the dynamics

Potential Energy Surface and Vibrational−Rotational Energy Levels of Hydrogen Peroxide

Abstract: The six-dimensional potential energy surface of hydrogen peroxide, H2O2, has been determined from large-scale ab initio calculations using the coupled-cluster method, CCSD(T), with the basis set of quadruple-ζ quality, cc-pVQZ. The effects of core-electron correlation on the calculated structural parameters and the torsional potential energy function have been investigated. The anharmonic quartic force field has been determined. The vibrational−rotational energy levels of the molecule have then been calculated using the variational method. The calculated molecular properties are found to be in good agreement with experimental data.

Sub-microhartree accuracy potential energy surface for H3 + including adiabatic and relativistic effects. II. Rovibrational analysis for H3 + and D3 +

Abstract: The 69 potential energy points of H3+ computed by Cencek et al. [J. Chem. Phys., 108, 2831 (1998), preceding paper] have been fitted to an analytical potential energy surface (PES). Rovibrational frequencies have been derived for the symmetric H3+ and D3+ isotopomers. A comparison with experiment shows residual discrepancies of a few tenths of cm−1 which can be ascribed mainly to nonadiabatic effects.

Ab initio potential energy surface and infrared spectra of H2-CO and D2-CO van der Waals complexes

Abstract: A new four-dimensional intermolecular potential energy surface for the H2-CO complex with fixed intramolecular distances of H2 and CO is presented. The symmetry-adapted perturbation theory has been used to calculate the interaction energy. A large basis set of spdfg type has been used including bond functions. An analytical fit of the ab initio potential energy surface has the global minimum of −109.272 cm−1 at the intermolecular separation of 7.76 bohr for the linear geometry with the C atom pointing toward the H2 molecule. This potential has been used to calculate rovibrational energy levels of CO-para-H2 and CO-ortho-D2 complexes. The resulting dissociation energies are 23.709 cm−1 and 30.756 cm−1, respectively. The computed levels have been used to generate the infrared spectrum accompanying the fundamental vibrational excitation of CO. The transition energies predicted agree well with those observed by McKellar [Chem. Phys. Lett. 186, 58 (1991)].

Transition‐state optimization on free energy surface: Toward solution chemical reaction ergodography

Abstract: To obtain a transition state (TS) in solution chemical reaction, a new TS optimization method has been proposed on a multidimensional free energy surface (FES). Analogous to the method for the Born–Oppenheimer potential energy surface using ab initio molecular orbital calculation, the present method utilizes force and Hessian on the FES, which can be calculated by molecular dynamics method and the free energy perturbation theory. Furthermore, on the basis of the method, we have proposed the definition of the intrinsic reaction coordinate (IRC) on the FES. According to not only the estimation of the computational demand but also the comparison of the numerical accuracy, we conclude that our method should be more efficient than such other methods that utilize only the free energy. Finally, it is discussed that the TS optimization and the IRC on the FES should become very important tools to develop a new research field called the solution chemical reaction ergodography. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 95–103, 1998

Crossed molecular beams and quasiclassical trajectory studies of the reaction O(1D)+H2(D2)

Abstract: The dynamics of the reactions O(1D)+H2→OH+H and O(1D)+D2→OD+D have been investigated in crossed molecular beam experiments with mass spectrometric detection at the collision energies of 1.9 and 3.0 kcal/mol, and 5.3 kcal/mol, respectively. From OH(OD) product laboratory angular and velocity distribution measurements, center-of-mass product translational energy and angular distributions were derived. The angular distributions are nearly backward–forward symmetric with a favored backward peaking which increases with collision energy. About 30% of the total available energy is found to be channeled into product translational energy. The results are compared with quasiclassical trajectory calculations on a DIM (diatomic-in-molecules) potential energy surface. Related experimental and theoretical works are noted. Insertion via the 1 1A′ ground state potential energy surface is the predominant mechanism, but the role of a second competitive abstraction micromechanism which should evolve on one of (or both) the first two excited surfaces 1A″ and 2 1A′ is called into play at all the investigated energies to account for the discrepancy between theoretical predictions and experimental results.

Molecular potential energy surfaces by interpolation in Cartesian coordinates

Abstract: We present a new method for expressing a molecular potential energy surface (PES) as an interpolation of local Taylor expansions. By using only Cartesian coordinates for the atomic positions, this method avoids redundancy problems associated with the use of internal coordinates. The correct translation, rotation, inversion, and permutation invariance are incorporated in the PES via the interpolation method itself. The method is most readily employed for bound molecules or clusters and is demonstrated by application to the vibrational motion of acetylene.

Study of Unimolecular Reactions by Coulomb Explosion Imaging: The Nondecaying Vinylidene

Abstract: The general situation of a bound system interacting with a system of much higher density of states appears in different fields of physics Some well known examples are doorway state reactions, giant resonances [1], radiationless transitions [2], and isomerization Often the production of “doorway states” is enabled by transitions which obey selection rules to proceed through a single resonant channel In the particular case of molecular isomerization, the process is sometimes described as follows A single state of an unstable (or metastable) isomer is created due to Franck-Condon overlap with an initial state Within a short time this state interacts with the vibrationally excited levels of the stable isomer, and the molecule undergoes internal radiationless rearrangement into the stable isomeric states This description is exact only at the limit of infinitely dense manifold of levels of the final states such as the dissociation into a continuum For many practical cases, the high density of the stable isomer states is approximated as such a continuum It is the purpose of this Letter to show how the Coulomb explosion imaging (CEI) method can be used to study such molecular isomerization by measuring the full nuclear density function of a molecule at a doorway energy state The structure and relative population of the isomers provides information on the nature of the isomeric interaction The equilibrium of the acetylene in the 1 S 1 ground electronic state has a linear (H—C — C—H) geometry and a rigid structure [3] Numerous theoretical studies of the potential energy surface [4‐ 9] predicted a local minimum at the “vinylidene” conformation ( :C— CH2) The energy of the vinylidene isomer is about 2 eV higher than the acetylene ground state, and the estimations of the isomerization barrier range from 0 to 026 eV Recent experimental evidence including this Letter support the existence of the vinylidene isomer Yet, the time dependent picture found in the literature is of an initially prepared vinylidene isomer which decays within a few picoseconds through a low barrier to acetylenic configurations Carrington et al [6] studied the vinylidene-acetylene isomerization dynamics by a semiclassical reaction path and analyzed it in terms of the vinylidene decay lifetime Their calculation yields a lifetime of 024 to 46 ps The vinylidene isomer can be accessed directly by photodetachment from the ground state of the negative ion which also possesses the vinylidene structure [10,11] This process has been investigated experimentally by Ervin and co-workers [12] who measured the corresponding photoelectron spectrum Pronounced peaks were resolved and assigned to the vinylidene normal mode vibrations Chen et al [13] measured a high resolution spectrum of acetylene in the energy region that corresponds to the predicted isomerization They were not able to assign the measured spectrum levels and used statistical analysis to attribute certain perturbations of the spectrum to the presence of the vinylidene isomer The experimental apparatus at the Weizmann Institute in Rehovot is designed for CEI measurements of neutral molecules The reader is referred to Ref [14] (and references therein) for a detailed description of the experimental setup and the principle of the CEI technique In the experiment reported here, C2H 2 anions were produced by

Global analytical potential hypersurfaces for large amplitude nuclear motion and reactions in methane. I. Formulation of the potentials and adjustment of parameters to ab initio data and experimental constraints

Abstract: Analytical representations of the global potential energy surface of XYn molecules are developed and applied to model the potential surface of methane in the electronic ground state. The generic analytical representation allows for a compact, robust, and flexible description of potentials for XYn systems irrespective of the specific nature of the atomic interactions. The functions are global in that structures near several minima of the potential hypersurface as well as saddle points and dissociation limits are well described. Clusters of atoms Yn can be represented as well by this type of function. Care is taken to implement conditions resulting from the symmetric group Sn and to construct positive definite bilinear forms of special functional forms of certain coordinates (such as bond lengths and bond angles), in order to avoid artifacts in exceptional ranges of the potential hypersurface. These special functional forms include intrinsic, symmetry allowed couplings between coordinates such as bending an...

Dissociative chemisorption of ch4 on ni : the role of molecular orientation

Abstract: The dissociation of CH4 on a Ni surface is examined quantum mechanically. The molecule is treated as a quasidiatomic R–H, where R=CH3, and vibrational, rotational, and translational motion normal to the surface are included. The metal surface is assumed to be flat and the potential energy surface is based on ab initio studies of dissociation over the Ni(111) atop site. Lattice motion is introduced via the surface mass model. Bessel–Legendre and fast Fourier transform pseudospectral techniques are used to evolve the wave function in time, and energy resolved reactive fluxes are extracted via a time-to-energy transform. Agreement with experiment is good, particularly with regard to the dependence of the dissociation probability on incident energy and surface temperature. The dynamics of the dissociation reaction for the various initial states of the molecule are examined.

Semiclassical initial value representation for rotational degrees of freedom: The tunneling dynamics of HCl dimer

Abstract: The semiclassical initial value representation (SC-IVR) is emerging as a practical way of generalizing classical trajectory simulation methods to include (approximately) the effects of quantum mechanics (i.e., interference and tunneling). This paper describes the application of the SC-IVR approach to determine the low lying vibrational states of the HCl dimer on a realistic potential energy surface. Overall agreement of the semiclassical energy levels with accurate quantum values is very good, including a good description of the tunneling splitting in the ground state. Issues regarding the applications of the SC-IVR methodology to the angular variables related to rotational degrees of freedom are explicitly discussed.

Origin of the Transition State on the Free Energy Surface: Intramolecular Proton Transfer Reaction of Glycine in Aqueous Solution

Abstract: Not only to elucidate the origin of the reaction barrier in liquid phase, i.e., the free energy of activation, but also to locate the proper transition state (TS) for a chemical reaction, the molecular dynamics method and the free energy perturbation theory have been applied to the intramolecular proton transfer reaction of glycine in aqueous solution, i.e., the zwitterion (ZW), to the neutral form (NF). The potential energy surface varies drastically as its environment changes from gas phase to aqueous solution, and experimentally, the existence of an entropy barrier is also suggested due to the solvent molecules. In this study, it is reported that the TS on the free energy surface (FES) corresponds approximately to the geometry at s ≈ 0.66 amu1/2 bohr, where s denotes the intrinsic reaction coordinate (IRC) for the gas-phase reaction, and therefore, the TS geometry is completely different from that for the gas phase. The free energy difference between ZW and NF is 8.46 ± 1.45 kcal/mol, and then the free...

Simple finite field method for calculation of static and dynamic vibrational hyperpolarizabilities: Curvature contributions

Abstract: In the static field limit, the vibrational hyperpolarizability consists of two contributions due to: (1) the shift in the equilibrium geometry (known as nuclear relaxation), and (2) the change in the shape of the potential energy surface (known as curvature). Simple finite field methods have previously been developed for evaluating these static field contributions and also for determining the effect of nuclear relaxation on dynamic vibrational hyperpolarizabilities in the infinite frequency approximation. In this paper the finite field approach is extended to include, within the infinite frequency approximation, the effect of curvature on the major dynamic nonlinear optical processes.