We present a new methodology to perform fast correlation-corrected vibrational self-consistent field (CC-VSCF) calculations using ab initio potential energy points calculated on the fly. Our method is based on the replacement of all-electron basis sets with a pseudo-potential basis for heavy atoms, and on an efficient reduction of the number of pair-coupling elements used in the CC-VSCF procedure. The method is applied to several test systems: H2O, NH3, and CH4, where it proves to be efficient, providing a speedup factor of 2 compared to a standard CC-VSCF calculation. We also apply our technique to the simulation of the vibrational spectrum of ethane and show that very accurate results can be obtained with a substantial speedup for this system.

Synthesis, structure and systematic study of the redox and spectroscopic properties of [{Mo₆X₈}(aromatic sulfonate)₆]²⁻.

Optimized geometries, potential well depths, and harmonic zero-point energies of the uracil–water, thymine–water, cytosine–water, and cytosine–(water)₂ weakly-bound molecules are computed using second-order Møller–Plesset perturbation theory and an interaction-optimized, singly-polarized double zeta basis set (DZPi). At the optimized geometries of the base–water structures, single point calculations are carried out using the slightly larger ESPB basis set, which is a singly-polarized ‘‘extended s ’’ basis set, containing a set of (s,p) bond functions at the midpoint of each hydrogen bond. All structures are also optimized with a simple intermolecular potential model, consisting of a Lennard–Jones repulsion–dispersion term and a point-charge model for the electrostatic interaction. The ab initio energies are used to assess the realism of the model potential for computing structures and frequencies within the harmonic approximation. The weakness of the harmonic approximation for these weakly bound complexes was assessed by using this potential in rigid-body diffusion Monte Carlo simulations to obtain the anharmonic zero-point energies and vibrationally averaged geometries of the molecular systems investigated. It is found that, although the anharmonicity correction to the zero-point energy is fairly small, the intermolecular bonds are significantly affected by vibrational averaging.

In this paper, we couple a numerical kinetic-energy operator approach to the direct-vibrational self-consistent field (VSCF)/vibrational configuration interaction (VCI) method for the calculation of vibrational anharmonic frequencies. By combining this with fast-VSCF, an efficient direct evaluation of the ab initio potential-energy surface (PES), we introduce a general formalism for the computation of vibrational bound states of molecular systems exhibiting large-amplitude motion such as methyl-group torsion. We validate our approach on an analytical two-dimensional model and apply it to the methanol molecule. We show that curvilinear coordinates lead to a significant improvement in the VSCF/VCI description of the torsional frequency in methanol, even for a simple two-mode coupling expansion of the PES. Moreover, we demonstrate that a curvilinear formulation of the fast-VSCF/VCI scheme improves its speed by a factor of two and its accuracy by a factor of 3.