Effect of ionic charge on energy contributions to the interaction of an ion with a polar molecule: ab initio calculations for M⋯H2O (M = Mg,Mg+, Mg2+, Ca,Ca+, Ca2+)

For M?H₂O (M = Mg, Mg⁺, Mg²⁺, Ca, Ca⁺, Ca²⁺) various energy contributions (first-order, induction and charge-transfer, dispersion) are compared. Near the minimum, stability due to the first-order energy decreases and that due to dispersion increases from M²⁺ to M⁰. For M²⁺, dispersion represents only 1–7% of the total energy; it may reach 25% with M⁺ and is largely responsible for stability of the neutral complex.     

Ab initio calculation of the first order term of the intermolecular energy near the van der Waals minimum

The first order term of the intermolecular energies between two hydrogen molecules and between Li⁺ and H₂ has been computed by three different methods: two of them are based on a perturbative procedure, including or neglecting the overlap between the orbitals of the interacting molecules or atoms in the calculation of the electrostatic and exchange terms. We can then study the effect of the overlap on each of these terms. The third method is the SCF supermolecule treatment which provides results in very good agreement with the perturbative procedure including the overlap. TheT configuration in the case of two hydrogen molecules and theC _2v configuration for Li⁺+H₂ are stable with respect to the first order term.     

Approximate theoretical determination of molecular static polarizabilities

Molecular polarizabilities are computed using ab initio SCF wave functions and second-order perturbation theory with special attention given to the use of a shifted denominator. Rather small basis sets are used, in order to obtain reasonable values at a reduced cost. Results are presented for H₂, CO, H₂O, C₂H₄, OCS, C₆H₆, Cl₂, Br₂, I₂.     

Dispersion energy in rare gas dimers from an ab initio perturbative procedure: Ne + Ne,Ar + Ar

The dispersion energy between two neon and two argon atoms is computed from an ab initio perturbative procedure not based on the multipole expansion. A comparison with the multipole expansion provides C₆ = 5.36 for Ne and 76.6 for Ar. It is seen that one d polarization function provides the main part of the C₆R^?6 contribution, the exponent of this function probably being related to the polarizability of the molecule. The multipole expansion seems acceptable up to the van der Waals minimum but quite invalid for smaller distances, and doubtful at the van der Waals minimum itself.     

Ab initio SCF-CI studies of the intermolecular interaction between two hydrogen molecules near the Van der Waals minimum

SCF-CI calculations have been used to study the intermolecular energy between two hydrogen molecules in four different geometrical configurations. The CI matrix was diagonalized using perturbation techniques. The importance of the perturbation expansion order upon the intermolecular energy could therefore be studied. The wave function includes all singly and doubly excited configurations. The natural orbitals have been determined and their relative importance on the intermolecular energy is considered.