An ab initio study of tunneling splittings in the water trimer

Tunneling splittings in the water trimer have been determined by the semiclassical WKB method, based on pathways characterized at the computational level of second-order M?ller-Plesset theory with basis sets of aug-cc-pVTZ quality. This calculation takes into account the single-flip and bifurcation tunneling rearrangements of the water trimer. The predicted splittings are 37.93 cm(-1) for the flip and 6.50x10(-3) cm(-1) for bifurcation, which agree quite well with the corresponding experimental values of 43.52 cm(-1) and 9.63x10(-3) cm(-1).     

An ab initio multireference perturbation theory study on the manganese dimer

The potential energy curves of the ground state and of some excited states of the manganese dimer have been calculated over a wide range of internuclear distances using the second order n-electron valence state perturbation theory applied to a complete active space self-consistent field reference wave function. The ground state of Mn(2), for which also the third order NEVPT has been used, is calculated to be a singlet belonging to the Sigma(g) (+) symmetry, characterized by a large equilibrium internuclear distance R(e) of 3.7-3.8 A, by a low dissociation energy D(e) of 0.07-0.08 eV, and by a small harmonic frequency omega(e) of 43 cm(-1). The experimental evidence that Mn(2) is a van der Waals molecule is thus confirmed. Among the excited states, (11)Pi(u), which is usually indicated as the ground state by density functional theory studies, appears as a low-lying state with R(e)=2.50 A, D(e)=1.35 eV, and omega(e)=246 cm(-1).     

Instanton calculations of tunneling splittings for water dimer and trimer

We investigate the ability of the recently developed ring-polymer instanton (RPI) method [J. O. Richardson and S. C. Althorpe, J. Chem. Phys. 134, 054109 (2011)] to treat tunneling in water clusters. We show that the RPI method is easy to extend to treat tunneling between more than two minima, using elementary graph theory. Tests of the method on water dimer and trimer yield a set of instanton periodic orbits which correspond to all known tunneling pathways in these systems. Splitting patterns obtained from the orbits are in good overall agreement with experiment. The agreement is closer for the deuterated than for the protonated clusters, almost certainly because the main approximation in the calculations is neglect of anharmonicity perpendicular to the tunneling path. All the calculations were performed on a desktop computer, which suggests that similar calculations will be possible on much larger clusters.     

An ab initio MO study on the thymine dimer and its radical cation

Ab initio SCF calculations with the 6-31G basis set for the thymine dimer (cys-syn form) and the thymine dimer radical cation are reported. The fusion of the thymine bases at the C5 and C6 positions involves the formation of a cyclobutane ring with puckering. The puckering causes a notable difference in the electronic structures of the two bases of the thymine dimer. The density of the HOMO orbital of the thymine dimer is localized on the O2, N1, and C6 atoms of both thymine rings, with the higher density on one of the rings. The HOMO orbital has a bonding character on the C6(SINGLEBOND)C6 bond. In the thymine dimer radical cation, the unpaired electron is localized mainly on the lengthened C6(SINGLEBOND)C6 bond with the higher density on one of the C6 atoms and to a lesser extent on the N1 atoms of both rings. © 1996 John Wiley & Sons, Inc.     

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 calculation on proton transfer in the benzoic acid dimer

We have made an ab initio calculation of the barriers for proton transfer in the hydrogen-bonded dimers of benzoic acid and acetic acid. Geometrical optimization values which are closer to the experiment one.     

A systematic ab initio study of the water dimer in hierarchies of basis sets and correlation models

A systematic, high-level ab initio investigation of the water dimer has been performed. The oxygen-oxygen bond distance has been estimated to be around 2.90 ?, about 0.05 ? shorter than the experimentally estimated distance, challenging the accuracy of the latter. The interaction energy has been obtained at −5.0±0.1 kcal/mol, which compares favourably with the experimentally estimated value of −5.4±0.7 kcal/mol. The importance of employing basis sets that include diffuse functions in correlated calculations on hydrogen-bonded systems is confirmed. In correlated calculations on the water dimer and the hydrogen fluoride dimer, the counterpoise-corrected interaction energies converge considerably slower towards the basis set limit than do the uncorrected energies, provided that the correlation-consistent basis sets are augmented with diffuse functions. Received: 12 February 1997 / Accepted: 5 June 1997     

An ab initio study of formamide

Calculated energy and molecular properties of the ground and low-energy excited states of formamide are presented at the ground state geometry. Satisfactory results are obtained except for the ¹* energy which remains too high by 1 eV (which is nevertheless a large improvement over previous calculations). The predicted triplet energies lie at 5.4 eV (³ n*) and 5.8 eV (³*).     

Interaction energy anisotropy of the pyrrole dimer: ab initio theoretical study

The van der Waals pyrrole dimer is studied using supermolecular and perturbation ab initio treatment with inclusion of correlation energy. The influence of selected geometry variations on the interaction energy components is investigated. Our calculations verified the minimum on the potential energy surface deduced from microwave spectra. Its stability is possibly related not to the extremal values of the selected interaction energy contributions but its physical origin is connected with the delicate equilibrium between the repulsive and attractive forces. Any structure variation connected with the extremal attraction energy is more than compensated for by the repulsion energy. Received: 11 June 1998 / Accepted: 6 October 1998 / Published online: 1 February 1999     

An ab initio study of the hydrated positron

Ab initio calculations are presented for the hydration energy of the positron. Tetrahedral molecular-dipole-oriented clusters e⁺(H₂O)₄ are considered. In performing these calculations, the Hartree—Fock MO LCAO SCF approximation with the 4-31G split-valence basis set is used. The method was modified to treat the positron problem. It is shown that e⁺ in liquid water, like an electron, can be strongly solvated, with the hydration energy 0.2–0.3 eV greater than that of e⁺.     

Accurate ab initio binding energies of the benzene dimer

Accurate binding energies of the benzene dimer at the T and parallel displaced (PD) configurations were determined using the single- and double-coupled cluster method with perturbative triple correction (CCSD(T)) with correlation-consistent basis sets and an effective basis set extrapolation scheme recently devised. The difference between the estimated CCSD(T) basis set limit electronic binding energies for the T and PD shapes appears to amount to more than 0.3 kcal/mol, indicating the PD shape is a more stable configuration than the T shape for this dimer in the gas phase. This conclusion is further strengthened when a vibrational zero-point correction to the electronic binding energies of this dimer is made, which increases the difference between the two configurations to 0.4-0.5 kcal/mol. The binding energies of 2.4 and 2.8 kcal/mol for the T and PD configurations are in good accord with the previous experimental result from ionization potential measurement.     

The intermolecular vibrations of the bifurcated water dimer: An Ab initio study

The intermolecular modes of the bifurcated water dimer are determined at the HF level using an extended basis set. In these computations, the donor libration frequency is found to be real and the bifurcated structure does not collapse toward the linear dimer. This result is contrary to all previous ab initio computations, which have predicted a Hessian with one negative eigenvalue. A good representation of other intermolecular modes, such as the libration of the acceptor, also requires an extended basis set. An interesting infrared active transition is predicted around 444 cm^?1. This transition, which corresponds to the donor wag, is found in the low-temperature spectrum of water in a N₂ matrix.     

An ab initio study of the keto-enol tautomerism

The keto-enol tautomerism is studied using an approximative HF method outlined in the appendix. The following results are obtained: (1) The experimentally observed alternance of G in acyclic monoketones could not be reproduced. (2) The stabilization of C=C double bonds, especially of conjugated double bonds, by CH₃- or -CH₂- groups is responsible for the observed difference between acyclic and cyclic 1.2-diketones, e.g. for the different enol content of diacetyl and cyclopentane-1.2-dione. (3) The enols of 1.2-diketones contain a hydrogen bond which differs from the hydrogen bond in enols of 1.3-diketones. (4) A system of two conjugated C=O double bonds is not favoured compared to a system of two C=O bonds which are separated by one (or more) -CH₂- group. (5) 5-ring enols with a C=C double bond in the ring are more stable than one would expect by an energy estimation from acyclic compounds.     

An ab initio study of ascorbic acid

The conformations of ascorbic acid and its singly ionized analog are found to agree with the X-ray structures. The calculations are discussed in terms of the known chemistry of ascorbic acid and its metabolites.     

Spectra of water dimer from a new ab initio potential with flexible monomers

We report the definition and testing of a new ab initio 12-dimensional potential for the water dimer with flexible monomers. Using our recent accurate CCpol-8s rigid water pair potential [W. Cencek, K. Szalewicz, C. Leforestier, R. van Harrevelt, and A. van der Avoird, Phys. Chem. Chem. Phys. 10, 4716 (2008)] as a reference for the undistorted monomers' geometries, a distortion correction has been added, which was taken from a former flexible-monomer ab initio potential. This correction allows us to retrieve the correct binding energy D(e)=21.0 kJ mol(-1), and leads to an equilibrium geometry in close agreement with the one obtained from benchmark calculations. The kinetic energy operator describing the flexible-monomer water dimer has been expressed in terms of Radau coordinates for each monomer and a recent general cluster polyspherical formulation describing their relative motions. Within this formulation, an adiabatic scheme has been invoked in order to decouple fast (intramolecular) modes and slow (intermolecular) ones. Different levels of approximation were tested, which differ in the way in which the residual potential coupling between the intramolecular modes located on different monomers and the dependence of the monomer rotational constants on the dimer geometry are handled. Accurate calculations of the vibration-rotation-tunneling levels of (H(2)O)(2) and (D(2)O)(2) were performed, which show the best agreement with experiments achieved so far for any water potential. Intramolecular excitations of the two monomers were calculated within two limiting cases, to account for the lack of non-adiabatic coupling between intramolecular modes due to the intermolecular motion. In the first model, the excitation was assumed to stay either on the donor or the acceptor molecule, and to hop between the two moieties upon donor-acceptor interchange. In the second model, the excitation remains on the same molecule whatever is the dimer geometry. Marginal frequency differences, less than 2?cm(-1), were obtained for all modes, and the resulting infrared shifts are in good agreement with experiments.     

Perturbative ab initio calculations of intermolecular energies. III. The water dimer

The interaction energy between two water molecules A and B is calculated by the method described in Paper I [1], previously applied for the interaction between two helium atoms (Paper II) [2]. This interaction energy is obtained as the difference between the energies of the complex (A + B) and the monomers (A) and (B), obtained by a perturbation method. The results obtained with the perturbation developed up to the second order in a minimal atomic basis set are decomposed into classical contributions and contributions linked to the exchange possibility. Charge transfer contributions are important and the localized character of the hydrogen bond is examined. It is pointed out that the definition of the set of excited configurations for the calculation of the energies of the isolated monomers is important, especially when one tries to use a small atomic basis set. A similar effect in SCF -type calculations is evaluated. The contribution of higher orders is evaluated by the CIPSI method.     

An ab initio study of pyruvic acid

The geometries and vibrational frequencies of two conformers of pyruvic acid have been obtained at the ab initio second order Möller-Plesset level of theory using the 6-311++G^** basis set. While the calculated geometries have been compared to the experimental microwave data, the vibrational frequencies have been assigned, using the experimental gas phase IR spectra of 13 isotopes of pyruvic acid by a recently developed scaling procedure (IRPROG). An attempt has been made to explain the stability of the eclipsed conformation over the staggered conformation of pyruvic acid by taking account of the molecular orbitals.     

An ab initio close-coupling calculation of the lower vibrational energies of the HCl dimer

We have previously determined an analytical ab initio six-dimensional potential energy surface for the HCl dimer, and in the present paper we use this potential, with the HCl bond lengths held fixed, in a full (four-dimensional) close-coupling calculation to determine the energies of the lowest 24 vibrational states. These vibrational states involve the intermolecular stretch ν₄, the trans-bend tunneling vibration ν₅, and the torsion ν₆. The highest of the 24 levels is the (ν₄ν₅ν₆)=(111) state, for which we calculate an energy of 200 cm⁻¹ above the (000) state. As well as determining tunneling energies up to 5ν₅=183 cm⁻¹, we determine ν₄=49 cm⁻¹, 2ν₄=93 cm⁻¹, 3ν₄=134 cm⁻¹, 4ν₄=172 cm⁻¹, ν₆=137 cm⁻¹ and ν₄+ν₆=178 cm⁻¹, together with tunneling energies in all these states. Making allowance for the HCl stretching zero-point energy we determine the dissociation energy D₀ as 390 cm⁻¹ on this analytical surface. We determine that below 300 cm⁻¹ there are 72 vibrational (J=K=0) states, and below dissociation there are 162 vibrational (J=K=0) states, for this potential surface.     

The intermolecular potentials of the O2-O2 dimer: a detailed ab initio study of the energy splittings for the three lowest multiplet states

Intermolecular potentials for the three lowest multiplet states (singlet, triplet and quintet) of the O(2)((3)Sigma)-O(2)((3)Sigma) dimer have been investigated in detail by means of high level ab initio calculations. The methods used include MRCI, ACPF, CASPT2, using different active spaces and basis sets. The results for the quintet state are compared with benchmark CCSD(T) calculations. As expected, the former methods do not account accurately for dispersion interactions, although the CASPT2 method performs better than the CI based ones. On the other hand, it is shown that highly correlated methods are necessary to accurately describe the splittings among the multiplet states. We propose to obtain singlet and triplet interaction potentials by combining CCSD(T) quintet potentials and multiconfigurational singlet-quintet and triplet-quintet splittings, respectively. The calculated splittings are quite stable regarding the method employed, except for the well region of the singlet and triplet states within the rectangular configuration, which corresponds to the absolute minima of these multiplet states. Nevertheless, we have been able to assess adequate upper and lower bounds to the interaction potential for this particular region.