Spin coupled valence bond theory of van der Waals systems: application to LiH … He

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Abstract
Spin coupled valence bond (SCVB) theory is applied to the study of weakly interacting systems. This ab initio approach is fully variational and it avoids carefully one of the main difficulties which plagues conventional molecular orbital based supermolecule calculations, namely the basis set superposition error. In addition, it proves possible to obtain a physically sound separation of the interaction energy into components such as the electrostatic, exchange, induction/polarization and dispersion contributions, in the same spirit as approaches based on perturbation theory. The SCVB approach is applied to the LiH … He interaction, with the LiH fragment fixed at its equilibrium geometry. The results highlight the accuracy of the method, relative to previous supermolecule studies. In agreement with previous work, the absolute minimum is found for the collinear approach of the helium atom to the lithium end of LiH. A small shallow secondary minimum, which might influence the dynamics at very low collision energies, is found for the other collinear approach, i.e., of helium to the Hδ- end of LiH.