Applications of a variational coupled-electron pair approach to the calculation of intermolecular interaction in the framework of the VB theory: Study of the van der Waals complex He-CH4

A general nonorthogonal coupled electron pair approach for the evaluation o f electron correlation contribution is presented in details. The self-consi stent field for molecular interactions wave function is used as reference s tate for a multistructure valence bond (VB) calculation. The central idea o f the method is the optimization of the virtual space of the VB wave functi on by means of a procedure very close to the independent electron pair appr oach (IEPA) scheme. All the orbitals employed are expanded in the basis set of their fragment so as to exclude the basis set superposition error (BSSE ) in a priori fashion. As an example, the application to the study of the v an der Waals complex He-CH4 is reported. The equilibrium geometry of the sy stem occurs at a He-C distance of 3.6 Angstrom , with the He atom pointing to the center of one of the faces of the CH4 molecule, with a well depth of 19 cm(-1). The potential energy surface of the He-CH4 complex is used to d etermine the parameters of a potential model which is employed in close-cou pling calculations of integral state-to-state cross sections for rotational ly inelastic scattering of methane molecules with helium atoms. The predict ed values are compared with the available experimental data. (C) 2000 Ameri can Institute of Physics. [S0021-9606(00)30732-2].