New basis sets for the evaluation of interaction energies: an ab initio study of the He-He, Ne-Ne, Ar-Ar, He-Ne, He-Ar and Ne-Ar van der Waals complex internuclear potentials and ro-vibrational spectra.

We investigated the performance of medium-size basis sets in calculations of interaction energies for small van der Waals complexes. After careful numerical tests we derived new efficient basis sets (modified LPol-n bases, MLPol-n) for the evaluation of the interaction energies for He-He, Ne-Ne, Ar-Ar, He-Ne, He-Ar and Ne-Ar complexes. Additionally, we carried out systematic basis set studies taking as a starting point Jensen's aug-pc-2 basis set. In both cases we considered the addition of midbond functions to the bases. The proposed MLPol-n sets use the model of a harmonic oscillator embedded in an external electric field and belong to the family of Pol bases. The quality of the MLPol-n sets was first tested through finite-field Hartree-Fock atomic polarizability calculations. Next we obtained coupled cluster singles and doubles (CCSD), including connected triples, CCSD(T), interaction potentials for the six van der Waals complexes, and taking as reference the most accurate theoretical values available in the literature, we implemented new basis sets for the evaluation of the interaction energies. To test the potentials further we used them to calculate the ro-vibrational spectra for the complexes. The results are in very good agreement with the experimental data. In the future, we plan to extend the study to larger van der Waals complexes.

The benzene-argon ground-state intermolecular potential energy surface revisited.

The benzene-Ar ground-state S(0) intermolecular potential energy surface is evaluated using the coupled cluster singles and doubles model including connected triple corrections and the augmented correlation consistent polarized valence triple-zeta basis set extended with a set of 3s3p2d1f1g midbond functions. The surface is characterized by absolute minima of -390.1 cm(-1) where the argon atom is located on the benzene C(6) axis at distances of +/-3.536 A, and has a general shape close to the available ground-state S(0) and the first singlet S(1) and triplet T(1) excited-state surfaces. Using the potential, the intermolecular level structure of the complex is evaluated. The new intermolecular potential energy surface gives very accurate results and improves those previously available.

The fluorobenzene-argon S(1) excited-state intermolecular potential energy surface.

We evaluate the first excited-state (S1) intermolecular potential energy surface for the fluorobenzene-Ar van der Waals complex using the coupled cluster method and the augmented correlation-consistent polarized valence double-zeta basis set extended with a set of 3s3p2d1f1g midbond functions. To calculate the S(1) interaction energies, we use ground-state interaction energies evaluated with the same basis set and the coupled cluster singles and doubles (CCSD) including connected triple excitations [CCSD(T)] model and interaction and excitation energies evaluated at the CCSD level. The surface minima are characterized by the Ar atom located above and below the fluorobenzene ring at a distance of 3.5060 A with respect to the fluorobenzene center of mass and at an angle of 5.89 degrees with respect to the axis perpendicular to the fluorobenzene plane. The corresponding interaction energy is -425.226 cm(-1). The surface is used in the evaluation of the intermolecular level structure of the complex, and the results are compared to the experimental data available and to those found in previous theoretical papers on ground-state potentials for similar complexes.