Showing papers on "Potential energy surface published in 1999"

A dimer method for finding saddle points on high dimensional potential surfaces using only first derivatives

Abstract: The problem of determining which activated (and slow) transitions can occur from a given initial state at a finite temperature is addressed. In the harmonic approximation to transition state theory this problem reduces to finding the set of low lying saddle points at the boundary of the potential energy basin associated with the initial state, as well as the relevant vibrational frequencies. Also, when full transition state theory calculations are carried out, it can be useful to know the location of the saddle points on the potential energy surface. A method for finding saddle points without knowledge of the final state of the transition is described. The method only makes use of first derivatives of the potential energy and is, therefore, applicable in situations where second derivatives are too costly or too tedious to evaluate, for example, in plane wave based density functional theory calculations. It is also designed to scale efficiently with the dimensionality of the system and can be applied to very large systems when empirical or semiempirical methods are used to obtain the atomic forces. The method can be started from the potential minimum representing the initial state, or from an initial guess closer to the saddle point. An application to Al adatom diffusion on an Al(100) surface described by an embedded atom method potential is presented. A large number of saddle points were found for adatom diffusion and dimer/vacancy formation. A surprisingly low energy four atom exchange process was found as well as processes indicative of local hex reconstruction of the surface layer.

Ab initio calculation of anharmonic vibrational states of polyatomic systems: Electronic structure combined with vibrational self-consistent field

Abstract: An algorithm for first-principles calculation of vibrational spectroscopy of polyatomic molecules is proposed, which combines electronic ab initio codes with the vibrational self-consistent field (VSCF) method, and with a perturbation-theoretic extension of VSCF. The integrated method directly uses points on the potential energy surface, computed from the electronic ab initio code, in the VSCF part. No fitting of an analytic potential function is involved. A key element in the approach is the approximation that only interactions between pairs of normal modes are important, while interactions of triples or more can be neglected. This assumption was found to hold well in applications. The new algorithm was applied to the fundamental vibrational excitations of H2O, Cl−(H2O), and (H2O)2, using the Moller–Plesset method for the electronic structure. The vibrational frequencies found are in very good accord with experiments. Estimates suggest that this electronic ab initio/VSCF approach should be feasible, with...

Structure and energetics of Ni, Ag, and Au nanoclusters

Abstract: The geometries and binding energies of the most stable isomers of nickel, silver, and gold nanoclusters of size 6, 7, 12, 13, 14, 19, 38, 55, and 75 atoms, predicted with an n-body Gupta potential, are presented. An exhaustive search for low-energy minima on the potential energy surface was carried out using an evolutive ~genetic-symbiotic! algorithm. Our results confirm the existence of disordered global minima for gold clusters of 19, 38, and 55 atoms in size, and disordered low-energy isomers for the 75-atom gold cluster. Disordered structures are also isomers of nickel and silver clusters but they are not among the global minima of these metals. Comparison of the structure factors of the disordered and ordered isomers of gold with published experimental x-ray powder diffraction data suggests that the disordered structures are real. The relation between the form of the n-body potential and the structure of the global minimum is studied, leading to an explanation of why these disordered states were located with the Gupta potential but not with certain other models of the metal bonding. @S0163-1829~99!05524-1#

Intermolecular potential of carbon dioxide dimer from symmetry-adapted perturbation theory

Abstract: A four-dimensional intermolecular potential energy surface for the carbon dioxide dimer has been computed using the many-body symmetry-adapted perturbation theory (SAPT) and a large 5s3p2d1f basis set including bond functions. The SAPT level applied is approximately equivalent to the supermolecular many-body perturbation theory at the second-order level. An accurate fit to the computed data has been obtained in a form of an angular expansion incorporating the asymptotic coefficients computed ab initio at the level consistent with the applied SAPT theory. A simpler site-site fit has also been developed to facilitate the use of the potential in molecular dynamics and Monte Carlo simulations. The quality of the new potential has been tested by computing the values of the second virial coefficient which agree very well with the experimental data over a wide range of temperatures. Our potential energy surface turns out to be substantially deeper than previous ab initio potentials. The minimum of −484 cm−1 has ...

van der waals interactions in the Cl + HD reaction

Abstract: The van der Waals forces in the entrance valley of the Cl + HD reaction are shown here to play a decisive role in the reaction9s dynamics. Exact quantum mechanical calculations of reactive scattering on a potential energy surface without Cl–HD van der Waals forces predict that the HCl and DCl products will be produced almost equally, whereas the same calculations on a new ab initio potential energy surface with van der Waals forces show a strong preference for the production of DCl. This preference is also seen in crossed molecular beam experiments on the reaction. The study of chemical reaction dynamics has now advanced to the stage where even comparatively weak van der Waals interactions can no longer be neglected in calculations of the potential energy surfaces of chemical reactions.

Atomic and molecular hydrogen interacting with Pt(111).

Abstract: This computational study is motivated by the apparent conflict between an experiment on dissociation of H2 and D2 on Pt(111), which suggests a rather corrugated potential energy surface (PES) for the H2/Pt(111) system, and an experiment showing only weak nonzero-order diffraction of HD scattering from Pt(111). In the calculations we have used density functional theory (DFT) within the generalized gradient approximation (GGA), including scalar relativistic effects and modelling the Pt(111) surface as a slab. We have found that the H2/Pt(111) PES is both energetically and geometrically corrugated. We have also found that there are reaction paths without or with very low barriers leading to dissociation of H2 on the Pt(111) surface, but that there are other reaction paths with substantial barriers. By performing extensive calculations on H interacting with a Pt(111) surface we have shown that a DFT/GGA approach that includes scalar relativistic effects is capable of describing the interaction between a hydro...

A new six-dimensional analytical potential up to chemically significant energies for the electronic ground state of hydrogen peroxide

Abstract: We report calculations of the electronic ground state potential energy surface (PES) of hydrogen peroxide covering, in an almost global fashion, all six internal degrees of freedom by two different ab initio techniques. Density functional theory (DFT) calculations using the Becke 3 parameter Lee-Yang-Parr (B3LYP) hybrid functional and multiconfigurational second order perturbation theory (CASPT2) calculations, both using large basis sets, are performed for a wide range of geometries (8145 DFT and 5310 CASPT2 single-point energies). We use a combined data set of mostly DFT with additional CASPT2 ab initio points and the complete CASPT2 surface to fit a total of four different 6D analytical representations. The resulting potentials contain 70-76 freely adjusted parameters and represent the ground state PES up to 40000 cm(-1) above the equilibrium energy with a standard deviation of 100-107 cm(-1) without any important artifacts. One of the model surfaces is further empirically refined to match the bond dissociation energy D-0 for HOOH --> 2OH. The potentials are designed for energy regions accessible by vibrational fundamental and overtone spectroscopy including the dissociation channel into hydroxyl radicals. Characteristic properties of the model surfaces are investigated by means of stationary point analyses, torsional barrier heights, harmonic frequencies, low-dimensional cuts and minimum energy paths for dissociation. Overall good agreement with high-level ab initio calculations, especially for the CASPT2 based potentials, is achieved. The drastic change in geometry at intermediate O-O distances, which reflects the transition from covalent to hydrogen bonding, is reproduced quantitatively. We calculate fully 6D anharmonic zero point energies and ground state torsional splittings with the diffusion quantum Monte Carlo method in perfect agreement, within statistical error bars, with experiment for the CASPT2 based potentials. Variational vibrational calculations in the (4+2)D adiabatic approximation yield energy levels and torsional splittings from the ground state up to predissociative states, satisfactorily reproducing the experimental transition wavenumbers. (C) 1999 American Institute of Physics. [S0021-9606(99)30205-1].

Semiclassical molecular dynamics simulations of excited state double-proton transfer in 7-azaindole dimers

Abstract: An ab initio excited state potential energy surface is constructed for describing excited state double proton transfer in the tautomerization reaction of photo-excited 7-azaindole dimers, and the ultrafast dynamics is simulated using the semiclassical (SC) initial value representation (IVR). The potential energy surface, determined in a reduced dimensionality, is obtained at the CIS level of quantum chemistry, and an approximate version of the SC-IVR approach is introduced which scales linearly with the number of degrees of freedom of the molecular system. The accuracy of this approximate SC-IVR approach is verified by comparing our semiclassical results with full quantum mechanical calculations. We find that proton transfer usually occurs during the first intermonomer symmetric-stretch vibration, about 100 fs after photoexcitation of the system, and produces an initial 15 percent population decay of the reactant base-pair, which is significantly reduced by isotopic substitution.

Quantum Mechanical Dynamical Effects in an Enzyme-Catalyzed Proton Transfer Reaction

Abstract: We have calculated the reaction rate and kinetic isotope effects for conversion of 2-phospho-d-glycerate to phosphoenolpyruvate by yeast enolase. The potential energy surface is modeled by a combined quantum mechanical/molecular mechanical method with generalized hybrid orbitals. The dynamics calculations are carried out by semiclassical variational transition state theory with multidimensional tunneling contributions. Quantum effects are included for a 25-atom cluster consisting of the substrate and part of the protein embedded in a rigid framework consisting of the rest of the protein and water. Quantum effects are important for calculating the absolute rate constant, and variational optimization of the dynamical bottleneck location is important for calculating the kinetic isotope effects. This provides the first evidence that transition state geometries are isotope dependent for enzyme reactions.

A quantum-mechanical study of the dynamics of the N(2D)+H2→NH+H reaction

Abstract: We have studied the low energy quantum dynamics of the N(2D)+H2(X 1Σg+)→NH(X 3Σ−)+H(2S) reaction. We use the hyperspherical method and a recently published ab initio potential energy surface. We find a forward–backward symmetry in the differential cross sections which is characteristic of a complex formation. We also present rotational and vibrational integral cross sections.

Atomistic modelling of BaTiO3 based on first-principles calculations

Abstract: Interatomic potentials are determined in the framework of a shell model used to simulate the structural instabilities, dynamical properties, and phase transition sequence of BaTiO3. The model is developed from first-principles calculations by mapping the potential energy surface for various ferroelectric distortions. The parameters are obtained by performing a fit of interatomic potentials to this energy surface. Several zero-temperature properties of BaTiO3, which are of central importance, are correctly simulated in the framework of our model. The phase diagram as a function of temperature is obtained through constant-pressure molecular dynamics simulations, showing that the non-trivial phase transition sequence of BaTiO3 is correctly reproduced. The lattice parameters and expansion coefficients for the different phases are in good agreement with experimental data, while the theoretically determined transition temperatures tend to be too small.

HCP CPH ISOMERIZATION: Caught in the Act

Abstract: ▪ Abstract In this overview we discuss the vibrational spectrum of phosphaethyne, HCP, in its electronic ground state, as revealed by complementary experimental and theoretical examinations. The main focus is the evolution of specific spectral patterns from the bottom of the potential well up to excitation energies of approximately 25,000 cm−1, where large-amplitude, isomerization-type motion from H–CP to CP–H is prominent. Distinct structural and dynamical changes, caused by an abrupt transformation from essentially HC bonding to mainly PH bonding, set in around 13,000 cm−1. They reflect saddle-node bifurcations in the classical phase space—a phenomenon well known in the nonlinear dynamics literature—and result in characteristic patterns in the spectrum and the quantum-number dependence of the vibrational fine-structure constants. Two polar opposites are employed to elucidate the spectral patterns: the exact solution of the Schrodinger equation, using an accurate potential energy surface and an effective...

Helicity decoupled quantum dynamics and capture model cross sections and rate constants for O(1D) + H2 → OH + H

Abstract: We study, within a helicity decoupled quantum approximation, the total angular momentum J dependence of reaction probabilities for the reaction O(1D)+H2→OH+H. A recently developed real wave packet approach is employed for the quantum calculations. The abinitio based, ground electronic (1A′) potential energy surface of Ho etal. (T-S. Ho, T. Hollebeeck, H. Rabitz, L. B. Harding and G. C. Schatz, J. Chem. Phys., 1996, 105, 10472) is assumed for most of our calculations, although some calculations are also performed with a recent surface due to Dobbyn and Knowles. We find that the low J reaction probabilities tend to be, on average, slightly lower than the high J probabilities. This effect is also found to be reproduced in classical trajectory calculations. A new capture model is proposed that incorporates the available quantum data within an orbital angular momentum or l-shifting approximation to predict total cross sections and rate constants. The results agree well with classical trajectory results and the experimental rate constant at room temperature. However, electronically non-adiabatic effects may become important at higher temperature.

Accurate structures and binding energies for small water clusters: The water trimer

Abstract: The global minimum on the water trimer potential energy surface has been investigated by means of second-order Mo/ller-Plesset (MP2) perturbation theory employing the series of correlation-consistent basis sets aug-cc-pVXZ (X = D, T, Q, 5, 6), the largest of which contains 1329 basis functions. Definitive predictions are made for the binding energy and equilibrium structure, and improved values are presented for the harmonic vibrational frequencies. A value of 15.82±0.05 kcal mol−1 is advanced for the infinite basis set frozen core MP2 binding energy, obtained by extrapolation of MP2 correlation energies computed at the aug-cc-pVQZ MP2 geometry. Inclusion of core correlation, using the aug-cc-pCV5Z basis set, has been found to increase the binding energy by 0.08 kcal mol−1, and after consideration of core correlation and higher-order correlation effects, the classical binding energy for the water trimer is estimated to be 15.9±0.2 kcal mol−1. A zero-point vibrational correction of −5.43 kcal mol−1 has bee...

Exploring the reaction dynamics of nitrogen atoms: A combined crossed beam and theoretical study of N(2D)+D2→ND+D

Abstract: In the first successful reactive scattering study of nitrogen atoms, the angular and velocity distribution of the ND product from the reaction N(2D)+D2 at 5.1 and 3.8 kcal/mol collision energies has been obtained in a crossed molecular beam study with mass spectrometric detection. The center-of-mass product angular distribution is found to be nearly backward–forward symmetric, reflecting an insertion dynamics. About 30% of the total available energy goes into product translation. The experimental results were compared with those of quasiclassical trajectory calculations on an accurate potential energy surface obtained from large scale ab initio electronic structure computations. Good agreement was found between the experimental results and the theoretical predictions.

Reliable theoretical treatment of molecular clusters: Counterpoise-corrected potential energy surface and anharmonic vibrational frequencies of the water dimer

Abstract: Structure, properties and energetics of the water dimer were determined by counterpoise (CP)-corrected gradient optimization which apriori eliminates the basis set superposition error (BSSE). Calculations were carried out at the MP2 level with various basis sets up to the aug-cc-pVQZ one. Besides harmonic vibrational frequencies twelve-dimensional anharmonic frequencies were also determined using the second-order perturbation treatment. Harmonic and anharmonic frequencies were based on CP-corrected Hessians. The equilibrium geometry of the dimer differs from that determined by a standard optimization and the difference becomes small only for the largest basis set (aug-cc-pVQZ). The best theoretical estimate of the intermolecular oxygen–oxygen distance (2.92 A) is shorter than the experimental result of 2.95 A. An estimate of the complete basis set limit of the stabilization energy was obtained by extrapolating the stabilization energies as a function of the reciprocal size of the basis set; this value (21.05 kJ mol-1) is slightly smaller than other literature estimates. Adding the changes due to zero-point energy and temperature-dependent enthalpy terms (determined using anharmonic frequencies obtained from the CP-corrected Hessian) we obtain an estimate to the theoretical stabilization enthalpy at 375 K (12.76 kJ mol-1) which is by 0.8–1.3 kJ mol-1 smaller than the literature results. Our theoretical value supports the very low limit of the experimental value. Red shift of the O–H stretching frequency accompanying formation of the dimer was determined at various theoretical levels and best agreement with the experimental value was found for anharmonic frequencies calculated with CP-corrected Hessians.

The Ground and Excited State Hydrogen Transfer Potential Energy Surface in 7-Azaindole

Abstract: Multireference wave functions, augmented by second-order perturbation theory, are used to examine the hydrogen transfer process in the ground and first excited states of 7-azaindole and in the 1:1 7-azaindole:water complex The presence of one water molecule dramatically reduces the barrier to proton transfer in both electronic states In the excited state the order of the two tautomers is reversed, and the barrier for the hydrogen transfer from the (now higher energy) normal structure to the tautomer in the presence of one water is estimated to be ≤6 kcal/mol

A four dimensional quantum scattering study of the Cl+CH4⇌HCl+CH3 reaction via spectral transform iteration

Abstract: We present a quantum dynamics study of the Cl+CH4⇌HCl+CH3 reaction using a four-dimensional rotating bond umbrella (RBU) model. A semiempirical potential energy surface is employed, where the zero point energy of the modes not explicitly treated in the RBU calculations is approximately included. The potential gives a vibrationally adiabatic ground state barrier height of 3.48 kcal/mol. The calculations have been performed in hypercylindrical coordinates using a log-derivative method. A single sector hyperspherical projection method has been developed for applying boundary conditions. A guided spectral transform (GST) Krylov subspace method has been constructed to find the eigenstates of the coupling matrix appearing in the coupled channel equations. The results show that the product methyl is rotationally cold for the forward reaction. A pronounced tunneling effect on the rate constants was obtained. The calculated thermal rate constants are 12%–45% smaller than the experimental results over the temperatu...

The dynamics of the hydrogen exchange reaction at 2.20 eV collision energy: Comparison of experimental and theoretical differential cross sections

Abstract: The H+D2(v=0,j=0)→HD(v′,j′)+D isotopic variant of the hydrogen atom exchange reaction has been studied in a crossed molecular beam experiment at a collision energy of 2.20 eV. Kinetic energy spectra of the nascent D atoms were obtained by using the Rydberg atom time-of-flight technique. The extensive set of spectra collected has permitted the derivation of rovibrationally state-resolved differential cross sections in the center-of-mass frame for most of the internal states of the HD product molecules, allowing a direct comparison with theoretical predictions. Accurate 3D quantum mechanical calculations have been carried out on the refined version of the latest Boothroyd–Keogh–Martin–Peterson potential energy surface, yielding an excellent agreement with the experimentally determined differential cross sections. The comparison of the results from quasi-classical trajectory calculations on the same potential surface reveals some discrepancies with the measured data, but shows a good global accordance. The t...

A first-principles potential energy surface for Eley–Rideal reaction dynamics of H atoms on Cu(111)

Abstract: We have performed first-principles total-energy calculations of low-dimensional sections of the electronically adiabatic potential energy surface (PES) that are relevant for the Eley–Rideal (ER) reaction of H atoms on a rigid Cu(111) surface. These calculations were performed within density-functional theory using a plane-wave and pseudopotential method and the generalized gradient approximation for the exchange-correlation energy. The calculated energy points for various configurations of one and two atoms on the Cu(111) surface were used to construct a model PES that can be used in ER reaction dynamics calculations.

A theoretical study of the vibrational energy spectrum of the hocl/hclo system on an accurate ab initio potential energy surface

Abstract: A new, global analytical potential energy surface is constructed for the X 1A′ electronic ground state of HOCl that accurately includes the HClO isomer. The potential is obtained by using accurate ab initio data from a previously published surface [Skokov et al., J. Chem. Phys. 109, 2662 (1998)], as well as a significant number of new data for the HClO region of the surface at the same multireference configuration interaction, complete basis set limit level of theory. Vibrational energy levels and intensities are computed for both HOCl and HClO up to the OH+Cl dissociation limit and above the isomerization barrier. After making only minor adjustments to the ab initio surface, the errors with respect to experiment for HOCl are generally within a few cm−1 for 22 vibrational levels with the largest error being 26 cm−1. A total of 813 bound vibrational states are calculated for HOCl. The HClO potential well supports 57 localized states, of which only the first 3 are bound. The strongest dipole transitions for...

The C7H10 Potential Energy Landscape: Concerted Transition States and Diradical Intermediates for the Retro-Diels−Alder Reaction and [1,3] Sigmatropic Shifts of Norbornene

Abstract: The potential energy surfaces for the thermal reactions of bicyclo[3.2.0]hept-2-ene and norbornene have been explored with density functional theory at the Becke3LYP/6-31G* level. Both concerted and diradical pathways for the retro-Diels−Alder reaction of norbornene have been examined, and the activation parameters and 13C primary kinetic isotope effects predicted for the concerted pathway are in excellent agreement with experimental data. The concerted mechanism is favored over the lowest energy stepwise diradical route by 12.4 kcal/mol. For the orbital symmetry-allowed suprafacial-inversion (si) pathway of the [1,3] sigmatropic rearrangement of bicyclo[3.2.0]hept-2-ene to form norbornene, a mechanism involving a transition state which leads to a broad diradical plateau on the potential energy surface is predicted. Implications of these surfaces, which differ substantially from those obtained by semiempirical calculations, are also discussed.

Ab initio ground potential energy surface, VTST and QCT study of the O(3P)+CH4(X 1A1)→OH(X 2Π)+CH3(X 2A2″) reaction

Abstract: An ab initio study of the ground potential energy surface (PES) of the O(3P)+CH4→OH+CH3 reaction has been performed using the second- and fourth-order Mo/ller–Plesset methods with a large basis set. A triatomic analytical ground PES with the methyl group treated as an atom of 15.0 a.m.u. has been derived. This PES has been employed to study the kinetics [variational transition state theory (VTST) and quasiclassical trajectory (QCT) rate constants] and dynamics (QCT method) of the reaction. The ab initio points have also been used directly to calculate the VTST rate constant considering all atoms of the system. The best VTST methods used lead to a good agreement with the experimental rate constant for 1000–2500 K, but QCT rate constant values are about one-third the experimental ones for 1500–2500 K. The cold QCT OH(v=0) rotational distribution arising from the simulation of the reaction with O(3P) atoms produced in the photodissociation of NO2 at 248 nm is in good agreement with experiment, while the very...

Benzene trimer and benzene tetramer: Structures and properties determined by the nonempirical model (NEMO) potential calibrated from the CCSD(T) benzene dimer energies

Abstract: The energetics and structure of the benzene trimer and tetramer are investigated with the nonempirical model (NEMO) potential calibrated to high precision by comparison with CCSD(T) benzene dimer energies. From the obtained potential energy surface, possible configurations could be determined and the experimental observed structures could be identified. This potential also reproduces the binding energies and allows for the determination of all intermolecular modes. It could be shown that this potential is therefore well suited and important to predict the structure and thermodynamic data for larger clusters, which cannot be accurately computed by ab initio quantum chemical methods.