Showing papers on "Potential energy surface published in 2000"

The Magnitude of the CH/π Interaction between Benzene and Some Model Hydrocarbons

Abstract: High-level ab initio calculations were carried out to evaluate the interaction between the π face of benzene and hydrocarbon molecules (methane, ethane, ethylene, and acetylene). Intermolecular interaction energies were calculated from extrapolated MP2 interaction energies at the basis set limit and CCSD(T) correction terms. The calculated benzene−methane interaction energy (−1.45 kcal/mol) is considerably smaller than that of the hydrogen bond between waters. The benzene−methane complex prefers a geometry in which the C−H bond points toward the benzene ring. The potential energy surface is very flat near the minimum, which shows that the major source of the attraction is a long-range interaction. The HF interaction energy of the complex (0.85 kcal/mol) is repulsive. The large gain of the attraction energy (−2.30 kcal/mol) by electron correlation correction indicates that dispersion interaction is the major source of the attraction. Although the electrostatic energy (−0.25 kcal/mol) is small, a highly ori...

Chemical content of the kinetic energy density

Abstract: A recent concept for the simulation of delocalized exact exchange [A.D. Becke, J. Chem. Phys. 112 (2000) 4020] involves a variable t σ that depends only on local values of the non-interacting kinetic energy density and of the charge density. Here, we show that this variable is highly indicative of details of atomic and molecular electronic structure, and reflects common chemical concepts such as atomic shells, molecular bonds and lone-pair regions, in a clear and intuitive manner. Explanations for this behavior are given in terms of localized orbitals, as well as on kinematic grounds. On the example of a few simple chemical reactions, it is demonstrated that bond formation and bond breaking are reflected in this variable, allowing a simple “sectioning” of the potential energy surface in terms of species involved.

Free energy calculation on enzyme reactions with an efficient iterative procedure to determine minimum energy paths on a combined ab initio QM/MM potential energy surface

Abstract: A new practical approach to studying enzyme reactions by combining ab initio QM/MM calculations with free energy perturbation is presented An efficient iterative optimization procedure has been developed to determine optimized structures and minimum energy paths for a system with thousands of atoms on the ab initio QM/MM potential: the small QM sub-system is optimized using a quasi-Newton minimizer in redundant internal coordinates with ab initio QM/MM calculations, while the large MM sub-system is minimized by the truncated Newton method in Cartesian coordinates with only molecular mechanical calculations The above two optimization procedures are performed iteratively until they converge With the determined minimum energy paths, free energy perturbation calculations are carried out to determine the change in free energy along the reaction coordinate Critical to the success of the iterative optimization procedure and the free energy calculations is the smooth connection between the QM and MM regions p

Computational Molecular Spectroscopy

Abstract: Partial table of contents: The Born--Oppenheimer Approximation (P. Bunker & P. Jensen). ELECTRONIC STATES. Ab Initio Determination of Accurate Ground Electronic State Potential Energy Hypersurfaces for Small Molecules (A. Csaszar, et al.). Symmetry Adapted Perturbation Theory Applied to the Computation of Intermolecular Forces (R. Moszynski, et al.). The Ab Initio Calculation of Molecular Properties Other than the Potential Energy Surface (S. Sauer & M. Packer). ROTATION--VIBRATION STATES. Perturbation Theory, Effective Hamiltonians and Force Constants (K. Sarka & J. Demaison). Variational Calculations of Rotation--Vibration Spectra (J. Tennyson). ROVIBRONIC STATES AND THE BREAKDOWN OF THE BORN--OPPENHEIMER APPROXIMATION. The Renner Effect (P. Jensen, et al.). The Renner--Teller Effect: The Effective Hamiltonian Approach (J. Brown). DYNAMICS. Forming Superposition States (T. Seideman). Ab Initio Molecular Dynamics (J. Tse & R. Rousseau). Index.

Origin of the Attraction and Directionality of the NH/π Interaction: Comparison with OH/π and CH/π Interactions

Abstract: High-level ab initio calculations were carried out to evaluate the interaction between the π face of benzene and ammonia as a model of NH/π interaction. The intermolecular interaction energy was calculated from the extrapolated MP2 interaction energy at the basis set limit and a CCSD(T) correction term. The calculated interaction energy (−2.22 kcal/mol) is considerably smaller than that of the hydrogen bond between waters. The monodentate complex is slightly more stable than the bidentate and tridentate complexes. The potential energy surface is very flat near the minimum, which shows that the major source of the attraction is a long-range interaction. The HF interaction energy of the monodentate complex (0.13 kcal/mol) is repulsive. The large gain in the attraction by electron correlation correction (−2.36 kcal/mol) indicates that the dispersion interaction is significantly important for the attraction. The electrostatic energy (−1.01 kcal/mol) is also important for the attraction. The benzene−water (OH/...

The mechanical strength of a covalent bond calculated by density functional theory

Abstract: The rupture forces of covalent bonds in a polymer as a function of bond lifetime are calculated with an Arrhenius kinetics model based on high-level density functional theory calculations. Relaxed potential energy surface scans of small model molecules yield potential functions that account for the deformations and hybridizations caused by the application of force. Morse potentials chosen to exhibit the same well depth and maximum force are used as an analytic representation of an individual bond in an infinitely long one-dimensional polymer. Application of force deforms the potential, and the activation energy for the bond rupture event together with the frequency of an optical phonon in the one-dimensional polymer are the two Arrhenius parameters. Rupture forces of the bonds C–C, C–N, C–O, Si–C, Si–N, Si–O, and Si–Si are reported as a function of the lifetime of the bond.

Dynamics of lennard-jones clusters: A characterization of the activation-relaxation technique

Abstract: The potential energy surface of Lennard-Jones clusters is investigated using the activation-relaxation technique (ART). This method defines events in the configurational energy landscape as a two-step process: (a) a configuration is first activated from a local minimum to a nearby saddle-point and (b) is then relaxed to a new minimum. Although ART has been applied with success to a wide range of materials such as a-Si, a-SiO2 and binary Lennard-Jones glasses, questions remain regarding the biases of the technique. We address some of these questions in a detailed study of ART-generated events in Lennard-Jones clusters, a system for which much is already known. In particular, we study the distribution of saddle-points, the pathways between configurations, and the reversibility of paths. We find that ART can identify all trajectories with a first-order saddle point leaving a given minimum, is fully reversible, and samples events following the Boltzmann weight at the saddle point.

Representation of the 6D potential energy surface for a diatomic molecule near a solid surface

Abstract: An efficient method is proposed to construct the six-dimensional Potential Energy Surface (PES) for diatomic molecule-surface interactions from low dimensional cuts obtained in ab initio calculations. The efficiency of our method results from a corrugation-reducing procedure based on the observation that most of the corrugation in a molecule-surface PES is already embedded in the atom-surface interactions. Hence, substraction of the latter leads to a much smoother function which makes accurate interpolations possible. The proposed method is a general one and can be implemented in a systematic way for any system. Its efficiency is illustrated for the case of H2/Pd(111) by using recent ab initio data. We report also the results of very stringent checks against ab initio calculations not used in the interpolation. These checks show the high accuracy of our method.

An accurate H2–H2 interaction potential from first principles

Abstract: We have calculated the potential energy surface extrapolated to the complete basis set limit using coupled-cluster theory with singles, doubles, and perturbational triples excitations [CCSD(T)] for the rigid monomer model of (H2)2. There is significant anisotropy among the 37 unique angular configurations selected to represent the surface. A four term spherical harmonics expansion model was chosen to fit the surface. The calculated potential energy surface reproduces the quadrupole moment to within 0.58% and the experimental well depth to within 1%. The second virial coefficient has been computed from the fitted potential energy surface. The usual semiclassical treatment of quantum mechanical effects on the second virial coefficient was applied in the temperature range of 100–500 K. We have developed a new technique for computing the quantum second virial coefficient by combining Feynman’s path integral formalism and Monte Carlo integration. The calculated virial coefficient compares very well with publis...

First-Principles Theory for the H + H2O, D2O Reactions

Abstract: A full quantum dynamical study of the reactions of a hydrogen atom with water, on an accurate ab initio potential energy surface, is reported. The theoretical results are compared with available experimental data for the exchange and abstraction reactions in H + D2O and H + H2O. Clear agreement between theory and experiment is revealed for available thermal rate coefficients and the effects of vibrational excitation of the reactants. The excellent agreement between experiment and theory on integral cross sections for the exchange reaction is unprecedented beyond atom-diatom reactions. However, the experimental cross sections for abstraction are larger than the theoretical values by more than a factor of 10. Further experiments are required to resolve this.

Dissociation and sticking of H 2 on the Ni(111), (100), and (110) substrate

Abstract: The adsorption, dissociation, and sticking of ${\mathrm{H}}_{2}$ on the Ni(111), (100), and (110) substrates are studied with spin-polarized gradient corrected density functional theory. To parametrize the six-dimensional (6D) potential energy surface (PES), between six and twelve two-dimensional sections of the PES are calculated using density functional theory. For the interpolation between such 2D sections, a scheme is developed and tested predicting the energy of the ${\mathrm{H}}_{2}$ molecules with an accuracy of about 50 meV in low-symmetry sites. On the interpolated 6D PES, classical simulations of the ${\mathrm{H}}_{2}$ sticking coefficient are performed, and the results are compared with experiment. The important experimental trends are well reproduced, and a simple model is discussed to explain why dissociation is activated on the (111) surface and nonactivated on the rough (110) surface. The results are compared to those for ${\mathrm{H}}_{2}$ on Pd, and it is shown that the difference between Ni and Pd stems mainly from the surface $s$ electrons.

Observation of a transition state resonance in the integral cross section of the F+HD reaction

Abstract: We have studied the reaction F+HD at low collision energies using a combination of experimental and theoretical methods. Clear evidence for a reactive resonance is found in the integral cross section for the reactive channel F+HD→HF+D. Using a crossed molecular beam apparatus, the total reactive cross sections for the HF+D and DF+H channels were obtained in the collision energy range of 0.2–5 kcal/mol. In addition, Doppler profiles were obtained over this range of energies, which provide information about the angularly resolved distribution of final vibrational states. The cross section shows a distinctive steplike feature near 0.5 kcal/mol. Furthermore, the Doppler profiles reveal a dramatic change in the angular distribution of products over a narrow energy range centered at 0.5 kcal/mol. This feature is shown to arise from a reactive resonance localized near the transition state. Theoretical scattering calculations have been carried out using the Stark–Werner potential energy surface, which accurately ...

Full dimensional quantum calculations of the CH4+H→CH3+H2 reaction rate

Abstract: Accurate full-dimensional quantum mechanical calculations are reported for the CH4+H→CH3+H2 reaction employing the Jordan–Gilbert potential energy surface. Benchmark results for the thermal rate constant and the cumulative reaction probability are presented and compared to classical transition state theory as well as reduced dimensionality quantum scattering calculations. The importance of quantum effects in this system is highlighted.

Time-resolved dissociative intense-laser field ionization for probing dynamics: Femtosecond photochemical ring opening of 1,3-cyclohexadiene

Abstract: The concerted photochemical ring opening of 1,3-cyclohexadiene was investigated in the gas phase by low-intensity pumping at 267 nm and subsequent probing by high-intensity photoionization at 800 nm and mass-selective detection of the ion yields. We found five different time constants which can be assigned to traveling times along consecutive parts of the potential energy surfaces. The molecule is first accelerated in the spectroscopic state 1B along Franck–Condon active coordinates, then alters direction before changing over to the dark state 2A. All constants including that for leaving the 2A surface are below 100 fs. These times are shorter than appropriate vibrational periods. Such a maximum speed is evidence that the pathway is continuous leading from surface to surface via real crossings (conical intersections) and that the molecule is accelerated right into the outlet of the 2A/1A funnel. On the ground state it arrives as a compact wave packet, indicating a certain degree of coherence. The experimental method promises a high potential for investigating dynamics, since many consecutive phases of the process can be detected. This is because the fragmentation pattern depends on the location on the potential energy surface, so that monitoring several different ions permits to conclude on the population flow through these locations. Ionization at the intensities used is normally considered to be an effect of the electric field of the radiation. But in our case it is enhanced by resonances in the neutral molecule and in particular in the singly positive ion, and it is not sensitive for the length of the molecule (different conformers of the product hexatriene). The ionic resonances explain why hexatriene has a much richer fragmentation pattern than cyclohexadiene. Coulomb explosion is observed from an excited state of a doubly positive ion. Its mechanism is discussed.

Quantum Dynamics of Hydride Transfer in Enzyme Catalysis

Abstract: One of the strongest experimental indications of hydrogen tunneling in biology has been the elevated Swain−Schaad exponent for the secondary kinetic isotope effect in the hydride-transfer step catalyzed by liver alcohol dehydrogenase. This process has been simulated using canonical variational transition-state theory for overbarrier dynamics and optimized multidimensional paths for tunneling. Semiclassical quantum effects on the dynamics are included on a 21-atom substrate−enzyme−coenzyme primary zone embedded in the potential of a substrate−enzyme−coenzyme−solvent secondary zone. The potential energy surface is calculated by treating 54 atoms by quantum mechanical electronic structure methods and 5506 protein, coenzyme, and solvent atoms by molecular mechanical force fields. We find an elevated Swain−Schaad exponent for the secondary kinetic isotope effect and generally good agreement with other experimental observables. Quantum mechanical tunneling is calculated to account for ∼60% of the reactive flux,...

Water pair potential of near spectroscopic accuracy. I. Analysis of potential surface and virial coefficients

Abstract: A new ab initio pair potential for water was generated by fitting 2510 interaction energies computed by the use of symmetry-adapted perturbation theory (SAPT). The new site–site functional form, named SAPT-5s, is simple enough to be applied in molecular simulations of condensed phases and at the same time reproduces the computed points with accuracy exceeding that of the elaborate SAPT-pp functional form used earlier [J. Chem. Phys. 107, 4207 (1997)]. SAPT-5s has been shown to quantitatively predict the water dimer spectra, see the following paper (paper II). It also gives the second virial coefficient in excellent agreement with experiment. Features of the water dimer potential energy surface have been analyzed using SAPT-5s. Average values of powers of the intermolecular separation—obtained from the ground-state rovibrational wave function computed in the SAPT-5s potential—have been combined with measured values to obtain a new empirical estimate of the equilibrium O–O separation equal to 5.50±0.01 bohr...

Quantum theory of bimolecular chemical reactions

Abstract: In this review we discuss quantum dynamically based theoretical methods for studying bimolecular gas phase chemical reactions. The scope is largely limited to reactions occurring on a single Born-Oppenheimer potential energy surface and mainly to time-independent Hamiltonians. An introductory overview is given, which includes a general discussion on approaches aiming to solve the time-independent and the time-dependent Schrodinger equation respectively, and how the resulting scattering matrix can be related to observables. The main topics of the review are the time-dependent wavepacket and the time-independent hyperspherical coordinate approaches to quantum dynamics. To perform such calculations it is necessary to first specify the relevant Hamiltonian operator, which has a kinetic and a potential part. The kinetic energy operator is easy to obtain in Cartesian coordinates, but more cumbersome in curvilinear coordinates. Two procedures are described. We also describe how to obtain the potential energy surfaces. The required input data are often obtained using ab initio methods, which themselves are not discussed. A very brief section on Green function methods for quantum scattering is also included. Rigorous quantum dynamic calculations quickly become expensive as the size of the problem grows. Therefore, reduced dimensionality calculations are often performed. However, only the most important degrees of freedom are treated explicitly. We discuss how the other degrees of freedom can be handled. An alternative way to reduce computational cost is to use semiclassical approaches, which have recently received renewed interest and we briefly review the main approaches. A few examples of reactions that have been studied with quantum dynamic theories are also included.

Potential energy surface, thermal, and state-selected rate coefficients, and kinetic isotope effects for Cl+CH4→HCl+CH3

Abstract: A new potential energy surface is reported for the gas-phase reaction Cl+CH4→HCl+CH3. It is based on the analytical function of Jordan and Gilbert for the analog reaction H+CH4→H2+CH3, and it is calibrated by using the experimental thermal rate coefficients and kinetic isotope effects. The forward and reverse thermal rate coefficients were calculated using variational transition state theory with semiclassical transmission coefficients over a wide temperature range, 200–2500 K. This surface is also used to analyze dynamical features, such as reaction-path curvature, the coupling between the reaction coordinate and vibrational modes, and the effect of vibrational excitation on the rate coefficients. We find that excitation of C–H stretching modes and Cl–H stretching modes enhances the rate of both the forward and the reverse reactions, and excitation of the lowest frequency bending mode in the CH4 reactant also enhances the rate coefficient for the forward reaction. However, the vibrational excitation of t...

The spin-conserved reaction CH+N2→H+NCN: A major pathway to prompt no studied by quantum/statistical theory calculations and kinetic modeling of rate constant

Abstract: A new spin-conserved path for the CH(2H)+N2 reaction at temperatures relevant to prompt NO formation has been theoretically investigated by means of ab initio MO calculations at the G2M level of theory. The result of the calculation reveals that the CH(2H)+N2 reaction takes place primarily via the ground electronic doublet potential energy surface, producing H+NCN instead of the commonly assumed, spin-forbidden HCN+N(4S) products. The overall rate constant for NCN production has been computed by a multichamel canonical variational Rice-Ramsperger-Kassel-Marcus theory calculation for the temperature range 1500–4000 K at 0.5–2 atm pressure: k3=2.22×107 T1.48 exp (−11760/T) cm3/(mol·s). The theoretically predicted rate constant was found to be in good agreement with high-temperature shock tube data kinetically modeled with the new mechahism that includes NCN reactions. In addition, k, was also found to be consistent with the apparent rate constants previously modeled for prompt NO formation in several flamer studies.

Finding transition structures in extended systems: A strategy based on a combined quantum mechanics–empirical valence bond approach

Abstract: A method for efficient localization and description of stationary points on the potential energy surface of extended systems is presented. It is based on Warshel’s empirical valence bond approach, for which we propose a modification, and combines the potential function description of the total system with a quantum mechanical description of the reaction site (QM-Pot). We describe the implementation of the method in the QMPOT program, which is basically an optimizer for minima and saddle points and has interfaces to existing quantum mechanical (e.g., TURBOMOLE, GAUSSIAN94) and interatomic potential function codes (e.g., GULP, DISCOVER). The power of the method is demonstrated for proton transfer reactions in zeolite catalysts, which may have as many as 289 atoms in the unit cell. As a test case the zeolite chabazite is considered in this study. Its limited unit cell size (37 atoms) makes comparison with the full periodic ab initio limit possible. The inclusion of long-range effects due to the periodic crys...

Multiconfiguration molecular mechanics algorithm for potential energy surfaces of chemical reactions

Abstract: We present an efficient algorithm for generating semiglobal potential energy surfaces of reactive systems. The method takes as input molecular mechanics force fields for reactants and products and a quadratic expansion of the potential energy surface around a small number of geometries whose locations are determined by an iterative process. These Hessian expansions might come, for example, from ab initio electronic structure calculations, density functional theory, or semiempirical molecular orbital theory. A 2×2 electronic diabatic Hamiltonian matrix is constructed from these data such that, by construction, the lowest eigenvalue of this matrix provides a semiglobal approximation to the lowest electronically adiabatic potential energy surface. The theory is illustrated and tested by applications to rate constant calculations for three gas-phase test reactions, namely, the isomerization of 1,3-cis-pentadiene, OH+CH4→H2O+CH3, and CH2Cl+CH3F→CH3Cl+CH2F.

Ab initio QM/MM simulations with a molecular orbital‐valence bond (MOVB) method: application to an SN2 reaction in water

Abstract: A mixed molecular orbital and valence bond (MOVB) method is described in combined ab initio QM/MM simulations of the SN2 reaction of Cl− + CH3Cl → ClCH3 + Cl− in water. The method is based on the construction of individual charge-localized, diabatic states using a block-localized wave function approach, followed by configuration interaction calculations to obtain the adiabatic potential energy surface. To examine the performance of the MOVB method, modern ab initio VB calculations were performed. The MOVB gas phase results are found to be in reasonable agreement in the overall potential energy surface in comparison with Hartree–Fock, MP2, and ab initio VB calculations. The only exception is that the activation energy is predicted to be about 4 kcal/mol higher in MOVB than in other methods. This is attributed to the configuration interaction procedure, which does not further optimize orbital coefficients in MOVB calculations, and it emphasizes the importance of orbital optimization in these calculations. The adiabatic ground-state potential surface can also be approximate by other quantum chemical models in Monte Carlo QM/MM simulations. At the HF/6-31G(d) level, the calculated activation free energy of 26 kcal/mol in water is in good agreement with experiment and with previous computational results. Importantly, the MOVB method allows for the solvent reaction coordinate to be used to define the reaction path in ab initio QM/MM simulations. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1458–1469, 2000

An ab initio molecular orbital-valence bond (MOVB) method for simulating chemical reactions in solution

Abstract: A mixed molecular orbital and valence bond (MOVB) method for describing the potential energy surface of reactive systems has been developed and applied to a model proton transfer reaction in aqueous solution. The MOVB method is based on a block-localized wave function (BLW) approach for defining the diabatic electronic states. Then, a configuration interaction Hamiltonian is constructed using these diabatic states as the basis function. It was found that the electronic coupling energy is large with a value of about 30 kcal/mol for the H3N−H−NH3+ system, whereas the predicted activation barrier is only 1.2 kcal/mol using the 3-21G basis set. The MOVB results are found to be in good accord with the corresponding ab initio Hartree−Fock calculations for the proton transfer process. We have also incorporated solvent effects into the MOVB Hamiltonian in the spirit of combined QM/MM calculations, and have modeled the proton transfer between ammonium ion and ammonia in water using Monte Carlo simulations. The pot...

Ab initio molecular dynamics studies of the photodissociation of formaldehyde, H2CO→H2+CO: Direct classical trajectory calculations by MP2 and density functional theory

Abstract: The dynamics of H2CO→H2+CO photodissociation have been studied by classical trajectory calculations at the MP2/6-311G(d,p), B3LYP/6-311G(d,p), and VSXC/6-311G(d,p) levels of theory. The trajectories were calculated directly from the electronic structure computations without first fitting a global potential energy surface. A Hessian based method with updating was used to integrate the trajectories. The translational energy distribution of the products is in better agreement with experiment than the previous Hartree–Fock direct trajectory calculations, since the MP2 and density functional methods reproduce the barrier height better. The MP2 and density functional calculations give very good descriptions of the product rotational state distributions and the CO vibrational state populations. The MP2 method yields a very good representation of the H2 vibrational state populations, whereas the density functional methods give too little H2 vibrational excitation and Hartree–Fock produces too much. This can be at...

A mobile charge densities in harmonic oscillators (MCDHO) molecular model for numerical simulations: The water-water interaction

Abstract: In this work we present a new proposal to model intermolecular interactions and use it for water molecules. The parameters of the model were fitted to reproduce the single molecule’s electrostatic properties, a sample of 352 points in a refined ab initio single molecule deformation potential energy surface (PES), and the theoretical limit of the dimerization energy, −20.8 kJ/mol. The model was able to reproduce a sample of 180 additional points in the single molecule deformation PES, and 736 points in a pair-interaction surface computed at the MP2/aug-cc-pVQZ′ level with the counterpoise correction. Though the model reproduced the diagonal of the polarizability tensor, it could account for only 60% of the three-body nonadditive contributions to the interaction energies in 174 trimers computed at the MP2/6-311++(2d,2p) level with full counterpoise correction, but reproduced the four-body nonadditivities in 34 tetramers computed at the same level as the trimers. The model’s predictions of the structures, energies, and dipoles of small clusters resulted in good agreement with experimental data and high quality ab initio calculations. The model also reproduced the second virial coefficient of steam at various temperatures, and the structure and thermodynamical properties of liquid water. We found that the short-range water–water interactions had a critical influence on the proper performance of the model. We also found that a model based on the proper intermolecular interactions requires the inclusion of intramolecular flexibility to be adequate.

Photochromism of salicylideneaniline (SA). How the photochromic transient is created: A theoretical approach

Abstract: The theoretical ab initio studies of the singlet states of salicylideneaniline (SA) are presented. The enol, cis-keto and trans-keto tautomers were treated by the HF/6-31G* (geometries and force fields of the ground states), and the CIS (excited states), methods. For the dynamic calculations of the rates of proton transfer (PT) in S1 states, the instanton approach was applied. It was found that the SA molecule in S0 and S1 states of both tautomers needs nonplanarity to stabilize. In the ground state the corresponding angle was calculated as 44° vs the experimental value, 49°. Upon twist of the excited system, the conical intersection of (π,π*) and (n,π*) potential surfaces takes place. In enol form the absolute minimum on the S1 potential energy surface belongs to a strongly twisted (n,π*) state. In keto-form this minimum corresponds to a planar (π,π*) state, while the twisted (n,π*) has the energy ≈1055 cm−1 higher. The angles of distortion are equal 93° and 80°, for the enol and keto form, respectively....

A detailed study on the symmetry breaking and its effect on the potential surface of NO3

Abstract: The tenacious symmetry breaking of the electronic wave function of the nitrate radical (NO3) and its effect on the ground-state potential energy surface is investigated in detail. The symmetry breaking of Hartree–Fock wave functions results from a dominance of the orbital localization effect over the resonance effect and leads to three different solutions, one symmetrical and two distorted ones, for the same electronic state. The respective equilibrium geometries of these solutions are points on different potential surfaces, making their comparison meaningless. The resonance effect is promoted by dynamic as well as static electron correlation. However, the dynamic correlation methods [e.g., many-body perturbation theory (MBPT) and coupled-cluster single double (CCSD)] cannot overcome the symmetry breaking of the reference function and the problem of multiple solutions persists. The symmetry breaking can be avoided by the complete active space self-consistent field (CASSCF) approach that yields unique, sin...