F2 dimer: Improved intermolecular potential energy surface using ab initio calculations

A computational study on the intermolecular potential energy of 44 different orientations of F₂ dimers is presented. Basis set superposition error (BSSE) corrected potential energy surface is calculated using the supermolecular approach at CCSD(T) and QCISD(T) levels of theory. The interaction energies obtained using the aug‐cc‐pVDZ and aug‐cc‐pVTZ basis sets are extrapolated to the complete basis set limit using the latest extrapolation scheme. The basis set effect is checked and it is found that the extrapolated intermolecular energies provide the best compromise between the accuracy and computational cost. Among 1320 energy points of F₂–F₂ system covering more relative orientations, the most stable structure of the dimers was obtained with a well depth of ?146.62 cm^?1 that related to cross configuration, and the most unstable structure is related to linear orientation with a well depth of ?52.63 cm^?1. The calculated second virial coefficients are in good agreement with experimental data. The latest extrapolation scheme of the complete basis set limit at the CCSD(T) level of theory is used to determine the intermolecular potential energy surface of the F₂ dimer. Comparing the results obtained by the latest scheme with those by older schemes show that the new approach provides the best compromise between accuracy and computational cost.     

Accurate ab initio intermolecular potential energy surface for the quintet state of the O2(3Sigma(g)-)-O2(3Sigma(g)-) dimer

A new potential energy surface (PES) for the quintet state of rigid O(2)((3)Sigma(g)(-)) + O(2)((3)Sigma(g)(-)) has been obtained using restricted coupled-cluster theory with singles, doubles, and perturbative triple excitations [RCCSD(T)]. A large number of relative orientations of the monomers (65) and intermolecular distances (17) have been considered. A spherical harmonic expansion of the interaction potential has been built from the ab initio data. It involves 29 terms, as a consequence of the large anisotropy of the interaction. The spherically averaged term agrees quite well with the one obtained from analysis of total integral cross sections. The absolute minimum of the PES corresponds to the crossed (D(2d)) structure (X shape) with an intermolecular distance of 6.224 bohrs and a well depth of 16.27 meV. Interestingly, the PES presents another (local) minimum close in energy (15.66 meV) at 6.50 bohrs and within a planar skewed geometry (S shape). We find that the origin of this second structure is due to the orientational dependence of the spin-exchange interactions which break the spin degeneracy and leads to three distinct intermolecular PESs with singlet, triplet, and quintet multiplicities. The lowest vibrational bound states of the O(2)-O(2) dimer have been obtained and it is found that they reflect the above mentioned topological features of the PES: The first allowed bound state for the (16)O isotope has an X structure but the next state is just 0.12 meV higher in energy and exhibits an S shape.     

An ab initio potential energy surface and vibrational energy levels of ZnH2

A three‐dimensional potential energy surface of the electronic ground state of ZnH₂ (${X}^1\sum _g^ +$ ) molecule is constructed from more than 7500 ab initio points calculated at the internally contracted multireference configuration interaction with the Davidson correction (icMRCI+Q) level employing large basis sets. The calculated relative energies of various dissociation reactions are in good agreement with the previous theoretical/experimental values. Low‐lying vibrational energy levels of ZnH₂, ZnD₂, and HZnD are calculated on the three‐dimensional potential energy surface using the Lanczos algorithm, and found to be in good agreement with the available experimental band origins and the previous theoretical values. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Theoretical studies of the N2O van der Waals dimer: ab initio potential energy surface, intermolecular vibrations and rotational transition frequencies

Theoretical studies of the potential energy surface and bound states were performed for the N(2)O dimer. A four-dimensional intermolecular potential energy surface (PES) was constructed at the CCSD(T) level with aug-cc-pVTZ basis set supplemented with bond functions. Three co-planar local minima were found on this surface. They correspond to a nonpolar isomer with slipped-antiparallel planar structure and two equivalent polar isomers with slipped-parallel planar structures. The nonpolar isomer is energetically more stable than the polar ones by 162 cm(-1). To assign the fundamental vibrational frequencies for both isomers, more than 150 vibrational bound states were calculated based on this PES. The orientation of the nodal surface of the wave functions plays an important role in the assignment of disrotation and conrotation vibrational modes. The calculated vibrational frequencies are in good agreement with the available experimental data. We have also found a quantum tunneling effect between the two equivalent polar structures in the higher vibrational excited states. Rotational transition frequencies of the polar structure were also calculated. The accuracy of the PES is validated by the good agreement between theoretical and experimental results for the transition frequencies and spectroscopic parameters.     

An ab initio potential energy surface and vibrational states of MgH2(1(1)A')

A three-dimensional global potential energy surface for the ground electronic state of MgH(2) is constructed from more than 3000 ab initio points calculated using the internally contracted multireference configuration interaction method with the Davidson correction at the complete basis set limit. Low-lying vibrational energy levels of MgH(2) and MgD(2) are calculated using the Lanczos algorithm, and found to be in good agreement with known experimental band origins. The majority of the vibrational energy levels up to 8000 cm(-1) are assigned with normal mode quantum numbers. However, our results indicate a gradual transition from a normal mode regime for the stretching vibrations at low energies to a local mode regime near 7400 cm(-1), as evidenced by a decreasing energy gap between the (n(1),0,0) and (n(1)-1,0,1) vibrational states and bifurcation of the corresponding wave functions.     

Intermolecular potential energy surface of the N2-CO dimer: ab initio investigation and analytical representation

In this work, for the first time, an analytical four-dimensional representation for the intermolecular potential of the N(2)-CO dimer is constructed from ab initio calculations. The most stable structure of dimer is found to be a distorted T-shape conformation with CO forming the top and N(2) the leg of T. Important structures of the dimer are characterized, and surprisingly, it is found that in contrast with general assumptions, the potential energy surface of the N(2)-CO dimer has a single symmetry unique minimum. The energy profile of a minimum energy path that connects two T-shaped saddle points to the minimum structure is derived. Important structures are characterized along this path to represent the concerted internal rotation of monomers within the complex. The second virial coefficient is calculated from the fitted PES, and reasonable agreement is found with recent experimental results.     

A new ab initio intermolecular potential energy surface and predicted rotational spectra of the Ne-H2S complex

We report a new three-dimensional ab initio intermolecular potential energy surface for the Ne-H(2)S complex with H(2)S monomer fixed at its experimental average structure. Using the supermolecular approach, the intermolecular potential energies were evaluated at CCSD(T) (coupled cluster with single and double and perturbative triple excitations) level with large basis sets including bond functions. The full counterpoise procedure was employed to correct the basis set superposition error. The planar T-shaped global minimum is located at the intermolecular distance of 3.51 ? with a well depth of 71.57 cm(-1). An additional planar local minimum was found to be separated from the global minimum with an energy barrier of 23.11 cm(-1). In addition, two first-order and one second-order saddle points were also located. The combined radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm were employed to evaluate the rovibrational energy levels for eight isotopic species of the Ne-H(2)S complexes. The rotational transition frequencies for the eight isotopomers were also determined for the ground and first vibrational excited states, which are all in very good agreement with the available experimental values.     

A new ab initio intermolecular potential energy surface and predicted rotational spectra of the Ar-H2S complex

We report a reliable three-dimensional ab initio intermolecular potential energy surface for the Ar-H(2)S complex with H(2)S monomer fixed at its experimental average structure. The potential energies were evaluated using the supermolecular approach at the coupled-cluster level with a large basis set including bond functions. The full counterpoise procedure was used to correct the basis set superposition error. The potential has a planar T-shaped global minimum with a well depth of 177.48 cm(-1) at the intermolecular distance of 3.72 ?. An additional planar local minimum is also found and is separated from the global minimum with an energy barrier with a height of 47.46 cm(-1). The combined radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm were employed to calculate the rovibrational energy levels for three isotopic species of Ar-H(2)S complexes (Ar-H(2)(32)S, Ar-H(2)(33)S, and Ar-H(2)(34)S). The rotational transition frequencies and structural parameters for the three isotopomers were also determined for the ground and the first excited states, which are all in good agreement with the available experimental values.     

Ab initio intermolecular potential energy surface of He-LiH

The intermolecular potential energy surface of He-LiH complex was studied using the full-electronic complete forth-order Miller-Plesset perturbation (MPPT) method.In ab initio calculations,the bond length of LiH was fixed at 0 159 5 nm.The potential has two local minima of Vm=-179.93 cm for the linear He LiH geormetrv at Rm=0.227 nm and Vm=-10.44 cm-1 for the linear He-HL1 geometry at Rm=0.516 nm The potemal exhibits strong anisotropy The analytic potential function with 31 parameters was determined by fitting to the calculated ab,mtio potentials The influence of variation of LiH bond length on the potential energy surface was also studied     

An ab initio potential energy surface and predissociative resonances of HArF

A three-dimensional potential energy surface of the ground electronic state HArF is constructed from more than 2000 ab initio points at the multireference averaged quadratic coupled-cluster level employing an augmented large basis set. The calculations indicate that the linear HArF molecule is metastable with a barrier of 0.643 eV in the atomization (HArF --> H + Ar + F) channel and a barrier of 1.017 eV in the dissociation (HArF --> Ar + HF) channel. Variational calculations of low-lying predissociative resonances of both HArF and DArF are performed on the three-dimensional potential energy surface using a complex-symmetric Lanczos propagation method, which yields both positions and widths of the resonance states. The resonance lifetime generally decreases with energy, but strong mode selectivity exists. Reasonably good agreement with experiment confirms the accuracy of our potential. These calculations provide valuable information on the stability and dynamics of HArF/DArF in its ground electronic state.     

A fully ab initio potential energy surface for ClH2 reactive system

An ab initio analytical potential energy surface called BW3 for the ClH2 reactive system is presented. The fit of this surface is based on about 1 200 ab initio energy points, computed with multi-reference configuration interaction(MRCI) and scaling external correlation (SEC) method and a very large basis set. The precision in the fit is very high. The BW3 surface could reproduce correctly the dissociation energy of H2 and HCl, and the endothermicity of the Cl + H2 abstraction reaction. For the Cl + H2 abstraction reaction, the saddle point of BW3 lies in collinear geometries, and the barrier height is 32.84 kJ/mol; for the H + ClH exchange reaction, the barrier of BW3 is also linear, with a height of 77.40 kJ/mol.     

An ab initio spin-orbit-corrected potential energy surface and dynamics for the F + CH4 and F + CHD3 reactions

We report an analytical ab initio three degrees of freedom (3D) spin-orbit-correction surface for the entrance channel of the F + methane reaction obtained by fitting the differences between the spin-orbit (SO) and non-relativistic electronic ground state energies computed at the MRCI+Q/aug-cc-pVTZ level of theory. The 3D model surface is given in terms of the distance, R(C-F), and relative orientation, Euler angles ? and θ, of the reactants treating CH(4) as a rigid rotor. The full-dimensional (12D) "hybrid" SO-corrected potential energy surface (PES) is obtained from the 3D SO-correction surface and a 12D non-SO PES. The SO interaction has a significant effect in the entrance-channel van der Waals region, whereas the effect on the energy at the early saddle point is only ～5% of that at the reactant asymptote; thus, the SO correction increases the barrier height by ～122 cm(-1). The 12D quasiclassical trajectory calculations for the F + CH(4) and F + CHD(3) reactions show that the SO effects decrease the cross sections by a factor of 2-4 at low collision energies and the effects are less significant as the collision energy increases. The inclusion of the SO correction in the PES does not change the product state distributions.     

An ab initio study of intermolecular interactions of nitromethane dimer and nitromethane trimer

Different geometries of nitromethane dimer and nitromethane trimer have been fully optimized employing the density functional theory B3LYP method and the 6-31++G** basis set. Three-body interaction energy has been obtained with the ab initio supermolecular approach at the levels of MP2/6-31++G**//B3LYP/6-31++G** and MP2/aug-cc-pVDZ//B3LYP/6-31++G**. The internal rotation of methyl group induced by intermolecular interaction has been observed theoretically. For the optimized structures of nitromethane dimer, the strength of C--H...O--N H-bond ranges from -9.0 to -12.4 kJ mol(-1) at the MP2/aug-cc-pVDZ//B3LYP/6-31++G** level, and the B3LYP method underestimates the interaction strength compared with the MP2 method, while MP2/6-31++G**//B3LYP/6-31++G** calculated DeltaE(C) is within 2.5 kJ mol(-1) of the corresponding value at the MP4(SDTQ)/6-31G**//B3LYP/6-31++G** level. The analytic atom-atom intermolecular potential has been successfully regressed by using the MP2/6-31++G**//B3LYP/6-31++G** calculated interaction energies of nitromethane dimer. For the optimized structures of nitromethane trimer the three-body interaction energies occupy small percentage of corresponding total binding energies, but become important for the compressed nitromethane explosive. In addition, it has been discovered that the three-body interaction energy in the cyclic nitromethane trimer is more and more negative as intermolecular distances decrease from 2.2 to 1.7 A.     

An ab initio correlated study of the potential energy surface for the HOBr.H2O complex

The potential energy surface (PES) for the HOBr.H(2)O complex has been investigated using second- and fourth-order M?ller-Plesset perturbation theory (MP2, MP4) and coupled cluster theory with single and doubles excitations (CCSD), and a perturbative approximation of triple excitations (CCSD-T), correlated ab initio levels of theory employing basis sets of triple zeta quality with polarization and diffuse functions up to the 6-311++G(3dp,3df ) standard Pople's basis set. Six stationary points being three minima, two first-order transition state (TS) structures and one second-order TS were located on the PES. The global minimum syn and the anti equilibrium structure are virtually degenerated [DeltaE(ele-nuc) approximately 0.3 kcal mol(-1), CCSD-T/6-311++G(3df,3pd) value], with the third minima being approximately 4 kcal mol(-1) away. IRC analysis was performed to confirm the correct connectivity of the two first-order TS structures. The CCSD-T/6-311++G(3df,3pd)//MP2/6-311G(d,p) barrier for the syn<-->anti interconversion is 0.3 kcal mol(-1), indicating that a mixture of the syn and anti forms of the HOBr.H(2)O complex is likely to exist.     

An intermolecular potential function for the Fe(II)–H2O system from ab initio calculations

An intermolecular potential for the Fe(II)–H₂O system has been determined from ab initio calculations which have been obtained with Huzinaga's MINI -2 basis set. Interaction energies for more than 100 points of the potential energy surface were fitted to an analytical function that contains 11 adjustable parameters. The goodness of the fitting and its applicability to the study of Fe(H₂O) clusters and to Monte Carlo simulations are discussed.     

Full-dimensional (15-dimensional) ab initio analytical potential energy surface for the H7+ cluster

Full-dimensional ab initio potential energy surface is constructed for the H(7)(+) cluster. The surface is a fit to roughly 160,000 interaction energies obtained with second-order M?llerPlesset perturbation theory and the cc-pVQZ basis set, using the invariant polynomial method [B. J. Braams and J. M. Bowman, Int. Rev. Phys. Chem. 28, 577 (2009)]. We employ permutationally invariant basis functions in Morse-type variables for all the internuclear distances to incorporate permutational symmetry with respect to interchange of H atoms into the representation of the surface. We describe how different configurations are selected in order to create the database of the interaction energies for the linear least squares fitting procedure. The root-mean-square error of the fit is 170 cm(-1) for the entire data set. The surface dissociates correctly to the H(5)(+) + H(2) fragments. A detailed analysis of its topology, as well as comparison with additional ab initio calculations, including harmonic frequencies, verify the quality and accuracy of the parameterized potential. This is the first attempt to present an analytical representation of the 15-dimensional surface of the H(7)(+) cluster for carrying out dynamics studies.     

A fully ab initio potential energy surface for C1H_2 reactive system

An ab initio analytical potential energy surface called BW3 for the CIH2 reactive system is presented. The fit of this surface is based on about 1 200 ab initio energy points, computed with multi-reference configuration interaction(MRCI) and scaling external correlation (SEC) method and a very large basis set. The precision in the fit is very high. The BW3 surface could reproduce correctly the dissociation energy of H2 and HCI, and the endothermicity of the Cl H2 abstraction reaction. For the Cl H2 abstraction reaction, the saddle point of BW3 lies in collinear geometries, and the barrier height is 32.84 kJ/mol; for the H CIH exchange reaction, the barrier of BW3 is also linear, with a height of 77.40 kJ/mol.     

New ab initio potential energy surface for the (HOCO+-He) van der Waals complex

A three-dimensional potential energy surface has been calculated for the ground electronic state of the HOCO+-He system. The calculations were performed at the coupled electron pair approximation level with an extended basis set which ensures a balance between accuracy and feasability. The validity of the method and of the basis set was tested through calculations of the polarizability of the He atom and of the spectroscopic constants of the HOCO+ ion. The calculated potential energy surface has been fitted to a spherical harmonic expansion to facilitate calculations of rotational excitation of HOCO+ by collisions with He.     

Correlation energy and dispersion interaction in the ab initio potential energy curve of the neon dimer

A close approximation to the empirical potential energy curve of the neon dimer is obtained by coupled-cluster singles plus doubles plus noniterative triples calculations by using nonaugmented correlation-consistent basis sets without counterpoise corrections and complementing them by three-term extrapolations to the complete basis set limit. The potential energy is resolved into a self-consistent-field Hartree-Fock contribution and a correlation contribution. The latter is shown to decay in the long-range region in accordance with the empirical dispersion expansion.