Features Of Potential Energy Surface Research Articles

The highly anharmonic BH5 potential energy surface characterized in the abinitiolimit

The strong sensitivity to level of theory of the salient features of the ground state potential energy surface of BH(5) has been overcome by rigorously converged ab initio computations employing correlation-consistent basis sets cc-p(C)VXZ (X=2-6), explicitly correlated R12 corrections, and coupled-cluster theory complete through quadruple excitations (CCSDTQ). Extrapolations via our focal point method yield a C(s)-symmetry global minimum of BH(3)-H(2) type featuring interfragment B-H distances of (1.401, 1.414) A, an H(2) bond length elongated to 0.803 A, and a BH(3)+H(2) dissociation energy D(e)(D(0))=6.6 (1.2) kcal mol(-1). The classical barriers for H(2) internal rotation and hydrogen scrambling are 0.07 and 5.81 kcal mol(-1), respectively, above the BH(5) minimum. Our thermochemical computations yield Delta(f)H(0) ( degrees )[BH(5)(g)]=-111.3+/-0.2 kcal mol(-1)+Delta(f)H(0) ( degrees )[B(g)], which is limited in accuracy only by persistent uncertainties in the enthalpy of formation of gaseous boron. As a first step in investigating the extremely anharmonic 12-dimensional vibrational dynamics of BH(5), a complete quartic force field has been computed at the all-electron cc-pCVQZ CCSD(T) level of theory. Previous matrix isolation infrared assignments of the nu(2) and nu(9) stretching modes of BH(5) compare favorably with our computed vibrational fundamentals, but the experimental assignment of the nu(6) bending mode of the BH(3) moiety is not supported by computed isotopic shifts.

Repulsive double many-body expansion potential energy surface for the reactions N(4S)+ H2⇌ NH(X3Σ–)+ H from accurate ab initio calculations

A single-sheeted DMBE potential energy surface is reported for the reactions N(4S)+H2<-->NH(X3Sigma-)+H based on a fit to accurate multireference configuration interaction energies. These have been calculated using the aug-cc-pVQZ basis set of Dunning and the full valence complete active space wave function as reference, being semi-empirically corrected by scaling the two-body and three-body dynamical correlation energies. The topographical features of the novel global potential energy surface are examined in detail, including a conical intersection involving the two first 4A'' potential energy surfaces which has been transformed into an avoided crossing in the present single-sheeted representation.

A theoretical study of cyclohexadiene/hexatriene photochemical interconversion: multireference configuration interaction potential energy surfaces and transition probabilities for the radiationless decays

The overall energetics and the feature of reactive potential energy surfaces for the photochemical interconversion between cyclohexadiene (CHD) and all- cis-hexatriene (cZc-HT) have been investigated using the multireference configuration interaction (MRCI) calculations. The adiabatic and the diabatic potential energy surfaces of the ground and the excited states have been calculated along the Jacobi coordinates. The conical intersections among the states are estimated and the corresponding non-adiabatic transition probabilities are calculated using the semiclassical Zhu–Nakamura formula. The 1 1B–2 1A decay occurs by C 2-symmetry-breaking motion around the conical intersection. The non-adiabatic transition to 1 1A occurs by the motion toward the 5-membered ring.

An ab initio study of the potential energy surfaces for the collision between a Cs atom and an I2 molecule

For the Cs + I2 collision system, a systematic theoretical study is first reported using the ab initio method. Three of eight possible channels are considered. The nonadiabatic coupling between the covalent state and the ionic one is calculated from different angles, especially the T-shape collision. The complete ion-pair formation potential energy surfaces of the T-shape collision in two electronic states (ionic 2B2 state and covalent 2A1 state) and the reactive surface of the linear collision are constructed at the QCISD(T)/SDD level. The main features of potential energy surfaces, such as the minimum energy reaction path, the crossing radius (Rc), and energy minimum geometries, are analyzed. The cross section of this titled system is calculated based on the harpoon mechanism and compared with the available experimental data and those obtained for the M + I2 (M = Li, Na) systems.Key words: ab initio two-state potential energy surfaces, nonadiabatic coupling, ion-pair formation, cross section.

Accurate Single-Valued Double Many-Body Expansion Potential Energy Surface for Ground-State HN2

A single-valued double many-body expansion potential energy surface is reported for ground-state HN2 by fitting accurate ab initio energies that have been suitably corrected by scaling for the H−N2 and N−NH regions. The energies of ∼900 geometries have been calculated at the multireference configuration interaction level, using aug-cc-pVQZ basis sets of Dunning and the full valence complete active space wave function as reference. The topographical features of the novel global potential energy surface are examined in detail.

A microscopic view of ion conduction through the K+ channel.

Recent results from x-ray crystallography and molecular dynamics free-energy simulations have revealed the existence of a number of specific cation-binding sites disposed along the narrow pore of the K+ channel from Streptomyces lividans (KcsA), suggesting that K+ ions might literally "hop" in single file from one binding site to the next as permeation proceeds. In support of this view, it was found that the ion configurations correspond to energy wells of similar depth and that ion translocation is opposed only by small energy barriers. Although such features of the multiion potential energy surface are certainly essential for achieving a high throughput rate, diffusional and dissipative dynamical factors must also be taken into consideration to understand how rapid conduction of K+ is possible. To elucidate the mechanism of ion conduction, we established a framework theory enabling the direct simulation of nonequilibrium fluxes by extending the results of molecular dynamics over macroscopically long times. In good accord with experimental measurements, the simulated maximum conductance of the channel at saturating concentration is on the order of 550 and 360 pS for outward and inward ions flux, respectively, with a unidirectional flux-ratio exponent of 3. Analysis of the ion-conduction process reveals a lack of equivalence between the cation-binding sites in the selectivity filter.

Rotational Isomerization of Dithienylethenes: A Study on the Mechanism Determining Quantum Yield of Cyclization Reaction

Aiming at the improvement of the photochemical performance of dithienylethenes, we present an analysis of the parameters that influence the quantum yield. We address some basic features of the ground-state potential energy surface of the photochromic dithienylethenes, showing the existence of three to five rotational conformers. Among those, only one is reactive. The energetical ordering and energy gaps between different conformers are investigated. The calculated results were compared to X-ray, proton NMR, and absorption spectra.

Conical intersections between three lowest adiabatic potential energy surfaces of the oxonium ion

In this Letter we present two conical intersection lines (CLs) between adiabatic potential energy surfaces (PESs) of oxonium correlated to the asymptotic systems H+ H 2 O +( X ) , H ++ H 2 O( X ) and H+ H 2 O +( A ̃ ) . Such PESs features are related to the dynamics of the charge transfer process H ++H 2O→H+H 2O + studied by Toennies and coworkers. A new three steps method is also presented whereby to determine the CLs. The generalized Heitler–London approximation is used first to obtain the analytical expression of a qualitative CL in whose neighborhood the accurate CL is finally ab initio computed after an intermediate searching step. Using this strategy the 3D configurational subspace where to search for crossing points is drastically reduced allowing for a faster determination of the CLs.

Theoretical Study on Mechanism of the 3CH2 + N2O Reaction

The complex triplet potential energy surface of the CH2N2O system, including 49 minimum isomers and 114 transition states, is investigated at the B3LYP and QCISD(T) (single-point) levels in order to explore the possible reaction mechanism of the 3CH2 radical with N2O. The most feasible pathway is the head-on attack of 3CH2 at the terminal N-atom of N2O to form cis-H2CNNO (a1) and trans-H2CNNO (a2). Both a1 and a2 can subsequently dissociate to give P1 (H2CN + NO) via the direct N−N bond rupture. Much less competitively, a1 can undergo a 1,4-H shift, leading to the chainlike isomer HCNNOH (k1), followed by the direct N−N bond cleavage to form product P2 (HCN + 3HON) or interconversion between the isomers k1−k8 and subsequent dissociation to P2. Furthermore, the products P1 (H2CN + NO) and P2 (HCN + 3HON) can undergo secondary dissociation to the same product P12 (HCN + NO + H). The formation of CO, however, seems impossible due to rather large barriers. Our results are in part contradictory with the recent time-resolved Fourier transform infrared spectroscopic study that nascent vibrationally excited products CO, NO, and HCN were observed. Since the initial N-attack step from R to a1 needs a considerable barrier of 14.8 kcal/mol, the title reaction may only be significant at high temperatures, as confirmed by the ab initio dynamic calculations on the rate constants. The reactivity discrepancies between the triplet and singlet CH2 with N2O are compared and discussed in terms of their potential energy surface features. Our calculations suggest that future experimental reinvestigations on the product distributions and rate constants of the title reaction at high temperatures are greatly desired.

Development of transferable interaction models for water. I. Prominent features of the water dimer potential energy surface

We present an analysis of the morphology of the water dimer potential energy surface (PES) obtained from ab initio electronic structure calculations and perform a quantitative comparison with the results from various water potentials. In order to characterize the morphology of the PES we have obtained minimum energy paths (MEPs) as a function of the intermolecular O–O separation by performing constrained optimizations under various symmetries (Cs, Ci, C2, and C2v). These constitute a primitive map of the dimer PES and aid in providing an account for some of its salient features such as the energetic stabilization of “doubly hydrogen-bonded” configurations for R(O–O)&amp;lt;2.66 Å. Among the various interaction potentials that are examined, it is found that the family of anisotropic site potential (ASP) models agrees better with the ab initio results in reproducing the geometries along the symmetry-constrained MEPs. It is demonstrated that the models that produce closest agreement with the morphology of the ab initio PES, tend to better reproduce the experimental data for the second virial coefficients. We finally comment on the functional forms of simple water models and discuss how effects such as charge overlap can be incorporated into such models.

A realistic double many-body expansion potential energy surface for [formula omitted] from a multiproperty fit to accurate ab initio energies and vibrational levels

A single-valued double many-body expansion potential energy surface (DMBE I) recently obtained for the ground electronic state of the sulfur dioxide molecule by fitting correlated ab initio energies suitably corrected by scaling the dynamical correlation energy is now refined by fitting simultaneously available spectroscopic levels up to 6886 cm −1 above the minimum. The topographical features of the novel potential energy surface (DMBE II) are examined in detail, and the method is emphasized as a robust route to fit together state-of-the-art theoretical calculations and spectroscopic measurements using a single fully dimensional potential form.

Theoretical Study on the Mechanism of the 1CHF + NO Reaction

The complex doublet potential energy surface of the CHFNO system is investigated at the QCISD(T)/6-311G(df,p)//B3LYP/6-311G(d,p) level in order to explore the possible reaction mechanism of 1CHF radical with NO. Twenty-six minimum isomers and fifty-nine transition states are located. Various possible reaction pathways are probed. It is shown that five dissociation products P1 HF + NCO, P2 F + HNCO, P4 OH + FCN, P5 F + HOCN, and P7 3NH + FCO are both thermodynamically and kinetically accessible. Among the five dissociation products, P2 and P4 may be the most abundant products with comparable quantities, whereas P1 is much less competitive followed by the almost negligible P5 and P7. Our results are in marked difference from previous experimental observation that only two dissociation products P1 and P2 are identified with the branching ratio being 6:4. However, and despite some energetic differences, our calculated potential energy surface features are quite in parallel to those of the analogous reaction 3CH2 + NO that has been extensively studied. Therefore, future experimental reinvestigations are desirable to clarify the mechanism of the title reaction. The present study may be useful for understanding the CHF chemistry.

A dynamic buckling geometric approach of 2-DOF autonomous potential lumped-mass systems under impact loading

Dynamic buckling for autonomous nondissipative lumped-mass systems under impact loading is thoroughly investigated. It is assumed that a fully plastic impact due to a striking body falling freely from a given height takes place, and that the effect of wave propagation can be neglected. Attention is focused on the post-impact dynamic buckling after establishing the initial velocities and the associated initial kinetic energy. Via a thorough discussion of the dynamic buckling mechanism based on certain salient geometric features of the total potential energy surface, one can obtain practically “exact” dynamic buckling loads, by extending previous findings valid for step load of infinite duration to the case of impact load. The proposed geometric approach, which is described for n-DOF systems and then presented in detail for 2-DOF systems, gives comprehensive, direct, readily obtained and reliable solutions compared to numerical integration schemes.

Intermolecular potential energy surface of the proton-bound H2O+–He dimer: Ab initio calculations and IR spectra of the O–H stretch vibrations

Rotationally resolved infrared photodissociation spectra of the O–H stretch fundamentals (ν1 and ν3) of the H2O+–He open shell ionic complex have been recorded in the doublet electronic ground state. The analysis of the complexation-induced frequency shifts (Δν1 = − 15 cm−1, Δν3 = − 5.1 cm−1) and the rotational structure is consistent with a planar, translinear proton-bound H–O–H–He equilibrium geometry. The derived intermolecular H–He separation and O–H–He bond angle are R0 = 1.756(4) A and φ0 = 175(5)° in the ground vibrational state. The experimental results are in good agreement with the ab initio calculations performed at the unrestricted MP2 level using a basis set of aug-cc-pVTZ quality. The global minimum on the calculated potential energy surface features a slightly translinear proton-bound equilibrium geometry, with an intermolecular separation of Re = 1.6990 A, a bond angle of φe = 173.2°, dissociation energies of De = 425.9 cm−1 and D0 = 158.6 cm−1, and frequency shifts of Δν1 = − 34.4 cm−1 and Δν3 = − 11.6 cm−1. Tunneling splittings in the perpendicular component of the ν3 band are attributed to hindered internal rotation exchanging the two equivalent proton-bound structures ia a planar transition state with C2v symmetry. The calculated barrier for this internal motion amounts to Vb = 202.9 cm−1 and the dimer is closer to the semirigid limit than to free internal rotation. The interpretation of the H2O+–He spectrum is supported by the corresponding spectrum of the monodeuterated HOD+–He complex.

Origin and Nature of Lithium and Hydrogen Bonds to Oxygen, Sulfur, and Selenium

Lithium and hydrogen bonded complexes of LiF and HF with H2CO, H2CS, and H2CSe have been investigated using higher level ab initio calculations. Extensive searches of the potential energy surfaces for equilibrium structures have been done at the Hartree−Fock level, and post Hartree−Fock calculations at MP2, MP4 levels and DFT calculations with B3LYP functional have been performed on the stable forms. 6-311++G(d,p) and 6-31++G(d,p) basis sets on H, C, O, and S and 6-311++G(d,p) basis set on Se have been employed throughout. NBO analysis of the wave functions have been done to trace the origin of various interactions that stabilize the complexes. Harmonic frequencies computed at Hartree−Fock level show that, of the 10 proposed structures, LiF and HF complexes have three and one stable forms, respectively. Potential energy surface features, structure, and stability of LiF complexes are completely different from those of HF complexes. Though it is commonly observed that lithium and hydrogen bonding interactions stabilize the complexes, the origin and nature of them is found to be different in each form and in each complex. This is well reflected in complex geometries and energetics.

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

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, significantly shorter than the previously accepted value. The residual errors in the SAPT-5s potential have been estimated by comparison to recent large-scale extrapolated ab initio calculations for water dimer. This estimate—together with the dissociation energy D0 computed from SAPT-5s—leads to a new prediction of the limit value of D0 equal to 1165±54 cm−1, close to but significantly more accurate than the best empirical value.

Monitoring laser driven hydrogen atom motion by transient infrared spectroscopy

Laser control of a hydrogen transfer reaction in a small model system embedded in a condensed phase environment is investigated. The two-dimensional model comprises those features of the multi-dimensional potential energy surface of thioacetylacetone which are most relevant for the isomerization reaction from the enol to the enethiol configuration. The influence of the remaining intramolecular coordinates as well as of a possible environment is described by a spectral density within the reduced density matrix approach. Different forms of the driving infrared laser pulse are explored placing emphasis on their capability for competing successfully with relaxation processes. The reaction dynamics is characterized by calculating the nonlinear optical response nonperturbatively in the driving field.

Rotational effects in the dissociation of H 2 on metal surfaces studied by ab initio quantum-dynamics calculations

By performing six-dimensional quantum-dynamical calculations on potential energy surfaces derived from density functional theory total-energy calculations, we have determined the sticking probability of H 2 on Rh(100), Pd(100) and Ag(100). In particular, we have focused on the dependence of the sticking probability on the initial rotational state of the impinging hydrogen beam. The dynamical results are related to features of the relevant potential energy surface.

Ab Initio Interaction Potentials for Simulations of Dimethylnitramine Solutions in Supercritical Carbon Dioxide with Cosolvents

Symmetry-adapted perturbation theory has been employed to calculate ab initio potential energy surfaces for complexes involved in the process of dissolving dimethylnitramine in supercritical carbon dioxide in the presence of acetonitrile or methyl alcohol as cosolvents. Site−site fits with correct asymptotic behavior have been developed for all potentials. The new potential energy surfaces can be used, along with the previously reported ones for carbon dioxide and carbon dioxide−acetonitrile dimers, in fully ab initio simulations of supercritical extraction processes. The physical interpretation of the features of the interaction potential energy surfaces resulting from the present approach challenges the established literature interpretations in terms of electrostatics only.

Ab initio MO calculations on potential energy surfaces of CCl4, is a C2v contact ion pair an isomer of Td–CCl4?

Abstract Ab initio molecular orbital (MO) calculations have been carried out for the ground-state potential energy surface of the CCl 4 molecule. It has been found that there is no local minimum corresponding to a C 2v contact ion CCl + 3 ···Cl − structure, which was previously reported to be an isomer of the most stable T d –CCl 4 molecule at the restricted Hartree–Fock level of theory. The primary failure of the previous study is that harmonic vibrational analyses as well as a check on instability for Hartree–Fock wavefunctions were not carried out. Configuration interaction calculations with single and double substitutions have been carried out to obtain qualitative features of excited-state potential energy surfaces and a local minimum corresponding to the ion pair structure has been found on the singlet excited-state surface.