Ab initio intermolecular potential energy surface and thermophysical properties of hydrogen sulfide

A six-dimensional potential energy hypersurface (PES) for two interacting rigid hydrogen sulfide molecules was determined from high-level quantum-mechanical ab initio computations. A total of 4016 points for 405 different angular orientations of two molecules were calculated utilizing the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory and extrapolating the calculated interaction energies to the complete basis set limit. An analytical site-site potential function with eleven sites per hydrogen sulfide molecule was fitted to the interaction energies. The PES has been validated by computing the second pressure virial coefficient, shear viscosity, thermal conductivity and comparing with the available experimental data. The calculated values of volume viscosity were not used to validate the potential as the low accuracy of the available data precluded such an approach. The second pressure virial coefficient was evaluated by means of the Takahashi and Imada approach, while the transport properties, in the dilute limit, were evaluated by utilizing the classical trajectory method. In general, the agreement with the primary experimental data is within the experimental error for temperatures higher than 300 K. For lower temperatures the lack of reliable data indicates that the values of the second pressure virial coefficient and of the transport properties calculated in this work are currently the most accurate estimates for the thermophysical properties of hydrogen sulfide.     

Vibrational energies of PH3 calculated variationally at the complete basis set limit

The potential energy surface for the electronic ground state of PH(3) was calculated at the CCSD(T) level using aug-cc-pV(Q+d)Z and aug-cc-pVQZ basis sets for P and H, respectively, with scalar relativistic corrections included. A parametrized function was fitted through these ab initio points, and one parameter of this function was empirically adjusted. This analytical PES was employed in variational calculations of vibrational energies with the newly developed program TROVE. The convergence of the calculated vibrational energies with increasing vibrational basis set size was improved by means of an extrapolation scheme analogous to the complete basis set limit schemes used in ab initio electronic structure calculations. The resulting theoretical energy values are in excellent agreement with the available experimentally derived values.     

Accurate Theoretical Study of LiS Radical and Its Singly Charged Cation and Anion in their Ground Electronic State

Potential energies of LiS(²Π), LiS^-(¹Σ⁺) and LiS⁺(³Σ^-) are calculated by using the multireference configuration interaction method including Davidson correction and the augmented correlation-consistent basis sets aug-cc-PV(X+d)Z (X=T, Q). Such obtained potential energies are subsequently extrapolated to the complete basis set limit. Both the core-valence correction and the relativistic effect are also considered. The analytical potential energy functions are then obtained by fitting such accurate energies utilizing a least-squares fitting procedure. By using such analytical potential energy functions, we obtain the accurate spectroscopic parameters, complete set of vibrational levels and classical turning points. The present results are compared well with the experimental and other theoretical work.     

Intermolecular potential energy surface for CS2 dimer

A new four‐dimensional intermolecular potential energy surface for CS₂ dimer is obtained by ab initio calculation of the interaction energies for a range of configurations and center‐of‐mass separation distances for the first time. The calculations were performed using the supermolecular approach at the Møller–Plesset second‐order perturbation (MP2) level of theory with the augmented correlation consistent basis sets (aug‐cc‐pVxZ, x = D, T) and corrected for the basis‐set superposition error using the full counterpoise correction method. A two‐point extrapolation method was used to extrapolate the calculated energy points to the complete basis set limit. The effect of using the higher levels of theory, quadratic configuration interaction containing single, double, and perturbative triple excitations QCISD(T) and coupled cluster singles, doubles and perturbative triples excitations CCSD(T), on the shape of potential energy surface was investigated. It is shown that the MP2 level of theory apparently performs extremely poorly for describing the intermolecular potential energy surface, overestimating the total energy by a factor of nearly 1.73 in comparison with the QCISD(T) and CCSD(T) values. The value of isotropic dipole–dipole dispersion coefficient (C₆) of CS₂ fluid was obtained from the extrapolated MP2 potential energy surface. The MP2 extrapolated energy points were fitted to well‐known analytical potential functions using two different methods to represent the potential energy surface analytically. The most stable configuration of the dimer was determined at R = 6.23 au, α = 90°, β = 90°, and γ = 90°, with a well depth of 3.980 kcal mol^?1 at the MP2 level of theory. Finally, the calculated second virial coefficients were compared with experimental values to test the quality of the presented potential energy surface. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011.     

Molecular dynamics simulations of fluid methane properties using ab initio intermolecular interaction potentials

Intermolecular interaction energy data for the methane dimer have been calculated at a spectroscopic accuracy and employed to construct an ab initio potential energy surface (PES) for molecular dynamics (MD) simulations of fluid methane properties. The full potential curves of the methane dimer at 12 symmetric conformations were calculated by the supermolecule counterpoise‐corrected second‐order Møller‐Plesset (MP2) perturbation theory. Single‐point coupled cluster with single and double and perturbative triple excitations [CCSD(T)] calculations were also carried out to calibrate the MP2 potentials. We employed Pople's medium size basis sets [up to 6‐311++G(3df, 3pd)] and Dunning's correlation consistent basis sets (cc‐pVXZ and aug‐cc‐pVXZ, X = D, T, Q). For each conformer, the intermolecular carbon–carbon separation was sampled in a step 0.1 Å for a range of 3–9 Å, resulting in a total of 732 configuration points calculated. The MP2 binding curves display significant anisotropy with respect to the relative orientations of the dimer. The potential curves at the complete basis set (CBS) limit were estimated using well‐established analytical extrapolation schemes. A 4‐site potential model with sites located at the hydrogen atoms was used to fit the ab initio potential data. This model stems from a hydrogen–hydrogen repulsion mechanism to explain the stability of the dimer structure. MD simulations using the ab initio PES show quantitative agreements on both the atom‐wise radial distribution functions and the self‐diffusion coefficients over a wide range of experimental conditions. © 2008 Wiley Periodicals, Inc. J Comput Chem 2009     

Estimated MP2 and CCSD(T) interaction energies of n-alkane dimers at the basis set limit: comparison of the methods of Helgaker et al. and Feller

The MP2 (the second-order M?ller-Plesset calculation) and CCSD(T) (coupled cluster calculation with single and double substitutions with noniterative triple excitations) interaction energies of all-trans n-alkane dimers were calculated using Dunning's [J. Chem. Phys. 90, 1007 (1989)] correlation consistent basis sets. The estimated MP2 interaction energies of methane, ethane, and propane dimers at the basis set limit [EMP2(limit)] by the method of Helgaker et al. [J. Chem. Phys. 106, 9639 (1997)] from the MP2/aug-cc-pVXZ (X=D and T) level interaction energies are very close to those estimated from the MP2/aug-cc-pVXZ (X=T and Q) level interaction energies. The estimated EMP2(limit) values of n-butane to n-heptane dimers from the MP2/cc-pVXZ (X=D and T) level interaction energies are very close to those from the MP2/aug-cc-pVXZ (X=D and T) ones. The EMP2(limit) values estimated by Feller's [J. Chem. Phys. 96, 6104 (1992)] method from the MP2/cc-pVXZ (X=D, T, and Q) level interaction energies are close to those estimated by the method of Helgaker et al. from the MP2/cc-pVXZ (X=T and Q) ones. The estimated EMP2(limit) values by the method of Helgaker et al. using the aug-cc-pVXZ (X=D and T) are close to these values. The estimated EMP2(limit) of the methane, ethane, propane, n-butane, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane dimers by the method of Helgaker et al. are -0.48, -1.35, -2.08, -2.97, -3.92, -4.91, -5.96, -6.68, -7.75, and -8.75 kcal/mol, respectively. Effects of electron correlation beyond MP2 are not large. The estimated CCSD(T) interaction energies of the methane, ethane, propane, and n-butane dimers at the basis set limit by the method of Helgaker et al. (-0.41, -1.22, -1.87, and -2.74 kcal/mol, respectively) from the CCSD(T)/cc-pVXZ (X=D and T) level interaction energies are close to the EMP2(limit) obtained using the same basis sets. The estimated EMP2(limit) values of the ten dimers were fitted to the form m0+m1X (X is 1 for methane, 2 for ethane, etc.). The obtained m0 and m1 (0.595 and -0.926 kcal/mol) show that the interactions between long n-alkane chains are significant. Analysis of basis set effects shows that cc-pVXZ (X=T, Q, or 5), aug-cc-pVXZ (X=D, T, Q, or 5) basis set, or 6-311G** basis set augmented with diffuse polarization function is necessary for quantitative evaluation of the interaction energies between n-alkane chains.     

Ab initio benchmark study for the oxidative addition of CH4 to Pd: importance of basis-set flexibility and polarization

To obtain a state-of-the-art benchmark potential energy surface (PES) for the archetypal oxidative addition of the methane C-H bond to the palladium atom, we have explored this PES using a hierarchical series of ab initio methods (Hartree-Fock, second-order M?ller-Plesset perturbation theory, fourth-order M?ller-Plesset perturbation theory with single, double and quadruple excitations, coupled cluster theory with single and double excitations (CCSD), and with triple excitations treated perturbatively [CCSD(T)]) and hybrid density functional theory using the B3LYP functional, in combination with a hierarchical series of ten Gaussian-type basis sets, up to g polarization. Relativistic effects are taken into account either through a relativistic effective core potential for palladium or through a full four-component all-electron approach. Counterpoise corrected relative energies of stationary points are converged to within 0.1-0.2 kcal/mol as a function of the basis-set size. Our best estimate of kinetic and thermodynamic parameters is -8.1 (-8.3) kcal/mol for the formation of the reactant complex, 5.8 (3.1) kcal/mol for the activation energy relative to the separate reactants, and 0.8 (-1.2) kcal/mol for the reaction energy (zero-point vibrational energy-corrected values in parentheses). This agrees well with available experimental data. Our work highlights the importance of sufficient higher angular momentum polarization functions, f and g, for correctly describing metal-d-electron correlation and, thus, for obtaining reliable relative energies. We show that standard basis sets, such as LANL2DZ+1f for palladium, are not sufficiently polarized for this purpose and lead to erroneous CCSD(T) results. B3LYP is associated with smaller basis set superposition errors and shows faster convergence with basis-set size but yields relative energies (in particular, a reaction barrier) that are ca. 3.5 kcal/mol higher than the corresponding CCSD(T) values.     

Large-amplitude dynamics in vinyl radical: the role of quantum tunneling as an isomerization mechanism

We report tunneling splittings associated with the large amplitude 1,2 H-atom migration to the global minima in the vinyl radical. These are obtained using a recent full-dimensional ab initio potential energy surface (PES) [A. R. Sharma, B. J. Braams, S. Carter, B. C. Shepler, and J. M. Bowman, J. Chem. Phys. 130(17), 174301 (2009)] and independently, directly calculated "reaction paths." The PES is a multidimensional fit to coupled cluster single and double and perturbative treatment of triple excitations coupled-cluster single double triple (CCSD(T)) with the augmented correlation consistent triple zeta basis set (aug-cc-pVTZ). The reaction path potentials are obtained from a series of CCSD(T)/aug-cc-pVnTZ calculations extrapolated to the complete basis set limit. Approximate 1D calculations of the tunneling splitting for these 1,2-H atom migrations are obtained using each of these potentials as well as quite different 1D Hamiltonians. The splittings are calculated over a large energy ranges, with results from the two sets of calculations in excellent agreement. Though negligibly slow (>1 s) for the vibrational ground state, this work predicts tunneling-promoted 1,2 hydride shift dynamics in vinyl to exhibit exponential growth with internal vibrational excitation, specifically achieving rates on the sub-μs time scale at energies above E ≈ 7500 cm(-1). Most importantly, these results begin to elucidate the possible role of quantum isomerization through barriers without dissociation, in competition with the more conventional picture of classical roaming permitted over a much narrower window of energies immediately below the bond dissociation limit. Furthermore, when integrated over a Boltzmann distribution of thermal energies, these microcanonical tunneling rates are consistent with sub-μs time scales for 1,2 hydride shift dynamics at T > 1400 K. These results have potential relevance for combustion modeling of low-pressure flames, as well as recent observations of nuclear spin statistical mixing from high-resolution IR/microwave spectroscopy on vinyl radical.     

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.     

Interactions in the B(+)-RG complexes and comparison with Be(+)-RG (RG = He-Rn): evidence for chemical bonding

Potential energy curves for the interaction of B(+) ((1)S) with RG ((1)S), RG = He-Rn, have been calculated at the CCSD(T) level of theory employing quadruple-ζ and quintuple-ζ quality basis sets. The interaction energies from these curves were subsequently point-by-point extrapolated to the basis set limit. Rovibrational energy levels have been calculated for each extrapolated curve, from which spectroscopic parameters are determined. These are compared to previously determined experimental and theoretical values. The potentials have also been employed to calculate the transport coefficients for B(+) traveling through a bath of RG atoms. We also investigate the interactions between B(+) and the rare gases via contour plots, natural population analysis (NPA), and molecular orbital diagrams. In addition, we consider the atoms-in-molecules (AIM) parameters. The interactions here are compared and contrasted with those for Li(+)-He and Be(+)-RG; it is concluded that there is significant and increasing dative covalent bonding for the Be(+)-RG and B(+)-RG complexes for RG = Ar-Rn, while the other species are predominantly physically bound.     

Experimental and theoretical study of the reaction of the ethynyl radical with nitrous oxide, C2H + N2O

We investigated the rate constants and reaction mechanism of the gas phase reaction between the ethynyl radical and nitrous oxide (C(2)H + N(2)O) using both experimental methods and electronic structure calculations. A pulsed-laser photolysis/chemiluminescence technique was used to determine the absolute rate coefficient over the temperature range 570 K to 836 K. In this experimental temperature range, the measured temperature dependence of the overall rate constants can be expressed as: k(T) (C(2)H + N(2)O) = 2.93 × 10(-11) exp((-4000 ± 1100) K/T) cm(3) s(-1) (95% statistical confidence). Portions of the C(2)H + N(2)O potential energy surface (PES), containing low-energy pathways, were constructed using the composite G3B3 method. A multi-step reaction route leading to the products HCCO + N(2) is clearly preferred. The high selectivity between product channels favouring N(2) formation occurs very early. The pathway corresponds to the addition of the terminal C atom of C(2)H to the terminal N atom of N(2)O. Refined calculations using the coupled-cluster theory whose electronic energies were extrapolated to the complete basis set limit CCSD(T)/CBS led to an energy barrier of 6.0 kcal mol(-1) for the entrance channel. The overall rate constant was also determined by application of transition-state theory and Rice-Ramsperger-Kassel-Marcus (RRKM) statistical analyses to the PES. The computed rate constants have similar temperature dependence to the experimental values, though were somewhat lower.     

The calculated infrared spectrum of Cl-H2O using a new full dimensional ab initio potential surface and dipole moment surface

We report a full dimensional, ab initio-based global potential energy surface (PES) and dipole moment surface for Cl-H2O. Both surfaces are symmetric with respect to interchange of the H atoms. The PES is a fit to thousands of electronic energies calculated using the coupled-cluster method [CCSD(T)] with a moderately large basis (aug-cc-pVTZ). Vibrational energies and wave functions are accurately obtained using MULTIMODE. The wave function and dipole moment surface are used to calculate and analyze the pure infrared spectrum at 0 K which is compared with experiment. Vibrational energies and the infrared spectra for DOD and HOD/DOH are also presented.     

Highly Accurate CCSD(T) and DFT–SAPT Stabilization Energies of H‐Bonded and Stacked Structures of the Uracil Dimer

The CCSD(T) interaction energies for the H‐bonded and stacked structures of the uracil dimer are determined at the aug‐cc‐pVDZ and aug‐cc‐pVTZ levels. On the basis of these calculations we can construct the CCSD(T) interaction energies at the complete basis set (CBS) limit. The most accurate energies, based either on direct extrapolation of the CCSD(T) correlation energies obtained with the aug‐cc‐pVDZ and aug‐cc‐pVTZ basis sets or on the sum of extrapolated MP2 interaction energies (from aug‐cc‐pVTZ and aug‐cc‐pVQZ basis sets) and extrapolated ΔCCSD(T) correction terms [difference between CCSD(T) and MP2 interaction energies] differ only slightly, which demonstrates the reliability and robustness of both techniques. The latter values, which represent new standards for the H‐bonding and stacking structures of the uracil dimer, differ from the previously published data for the S22 set by a small amount. This suggests that interaction energies of the S22 set are generated with chemical accuracy. The most accurate CCSD(T)/CBS interaction energies are compared with interaction energies obtained from various computational procedures, namely the SCS–MP2 (SCS: spin‐component‐scaled), SCS(MI)–MP2 (MI: molecular interaction), MP3, dispersion‐augmented DFT (DFT–D), M06–2X, and DFT–SAPT (SAPT: symmetry‐adapted perturbation theory) methods. Among these techniques, the best results are obtained with the SCS(MI)–MP2 method. Remarkably good binding energies are also obtained with the DFT–SAPT method. Both DFT techniques tested yield similarly good interaction energies. The large magnitude of the stacking energy for the uracil dimer, compared to that of the benzene dimer, is explained by attractive electrostatic interactions present in the stacked uracil dimer. These interactions force both subsystems to approach each other and the dispersion energy benefits from a shorter intersystem separation.     

Three-dimensional Potential Energy Surface and Bound States of the Ar2-Ne Complex

The first three-dimensional interaction potential energy surface (PES) of the Ar₂-Ne complex is developed using the single and double excitation coupled cluster theory with noniterative treatment of triple excitations CCSD(T). The aug-cc-pVQZ basis sets are employed for all atoms, including an additional (3s3p2d2f1g) set of midpoint bond functions. The calculated single point energies are fitted to an analytic two-dimensional potential model at each of seven fixed r_Ar2 values. The seven model potentials are then used to construct the three-dimensional PES by interpolating along (r—r_e) using a sixth-order polynomial. The PES is used in the following rovibrational energy levels calculations. The comparisons of theoretical transition frequencies and spectroscopic constants with the experimental results are given.     

Do B3LYP and CCSDT predict different hydrosilylation mechanisms? Influences of theoretical methods and basis sets on relative energies in ruthenium-silylene-catalyzed ethylene hydrosilylation

A series of density functional theory (DFT) and wave function theory (WFT) methods were used in conjunction with a series of basis sets to investigate the influence of the computational methodology on the relative energies of key intermediates and transition states in potential reaction pathways in ruthenium-silylene-catalyzed hydrosilylation reactions. A variety of DFT methods in a modest basis set and B3LYP calculations in a variety of basis sets calculated the key transition in the Glaser-Tilley (GT) pathway to be energetically favored. In contrast, with the smaller basis sets, the CCSD(T) method calculated the Chalk-Harrod (CH) pathway to be favored; however, CCSD(T) results extrapolated to larger basis sets favored the GT pathway.     

New ab initio potential energy surface for BrH2 and rate constants for the H + HBr → H2 + Br abstraction reaction

A global potential energy surface (PES) for the electronic ground state of the BrH(2) system was constructed based on the multireference configuration interaction (MRCI) method including the Davidson's correction using a large basis set. In addition, the spin-orbit correction were computed using the Breit-Pauli Hamiltonian and the unperturbed MRCI wavefunctions in the Br + H(2) channel and the transition state region. Adding the correction to the ground state potential, the lowest spin-orbit correlated adiabatic potential was obtained. The characters of the new potential are discussed. Accurate initial state specified rate constants for the H + HBr → H(2) + Br abstraction reaction were calculated using a time-dependent wave packet method. The predicted rate constants were found to be in excellent agreement with the available experimental values and much better than those obtained from a previous PES.     

A new "spectroscopic" potential energy surface for formaldehyde in its ground electronic state

We report a new "spectroscopic" potential energy surface (PES) of formaldehyde (H(2)(12)C(16)O) in its ground electronic state, obtained by refining an ab initio PES in a least-squares fitting to the experimental spectroscopic data for formaldehyde currently available in the literature. The ab initio PES was computed using the CCSD(T)/aug-cc-pVQZ method at 30 840 geometries that cover the energy range up to 44 000 cm(-1) above equilibrium. Ro-vibrational energies of formaldehyde were determined variationally for this ab initio PES by means of the program TROVE [Theoretical ROtation-Vibration Energies; S. N. Yurchenko, W. Thiel, and P. Jensen, J. Mol. Spectrosc. 245, 126 (2007)]. The parameter values in the analytical representation of the PES were optimized in fittings to 319 ro-vibrational energies with J = 0, 1, 2, and 5. The initial parameter values in the fittings were those of the ab initio PES, the ro-vibrational eigenfunctions obtained from this PES served as a basis set during the fitting process, and constraints were imposed to ensure that the refined PES does not deviate unphysically from the ab initio one in regions of configuration space not sampled by the experimental data. The resulting refined PES, referred to as H(2)CO-2011, reproduces the available experimental J ≤ 5 data with a root-mean-square error of 0.04 cm(-1).     

The calculated infrared spectrum of Cl- H2O using a full dimensional ab initio potential surface and dipole moment surface

We report a full dimensional ab initio-based global potential energy surface (PES) and dipole moment surface (DMS) for Cl(-)H(2)O. Both surfaces are symmetric with respect to interchange of the H atoms. The PES is a fit to thousands of electronic energies calculated using the coupled-cluster method (CCSD(T)) with a moderately large basis (aug-cc-pVTZ). The infrared spectrum and vibrational dynamics are reported and compared to experiment. These results are analyzed by examination of wave function and the dipole surface.     

Basis set effects on the intermolecular interaction of the H2-H2 system obtained using ab initio molecular orbital calculations with the Møller-Plesset perturbation correction

The intermolecular interaction potential of the H₂-H₂ system was calculated by an ab initio molecular orbital method using several basis sets (up to 6-31 lG(3pd)) with inclusion of the electron correlation correction of the Møller-Plesset perturbation method and the basis set superposition error (BSSE) correction of the counterpoise method in order to evaluate the basis set effect. The calculated interaction energies depend strongly on the basis set used. Whereas the interaction energies of the repulsive and coulombic energy components calculated at the Hartree-Fock level are not affected by a change of basis set, the dispersion energy component depends strongly on the basis set used. Parameters of an exp-6-1 type non-bonding interaction potential were optimized on the basis of the MP4(SDTQ)/6-311G(3p) level intermolecular interaction energies of the H₂-H₂ system.