Abstract
A new approach to the calculation of intermolecular forces is presented. The total hamiltonian is used and no arbitrary separation into “orders” is made. The approach allows the interaction to be calculated, including intraatomic electron correction, to various degrees of approximation, the ultimate being full configuration interaction. The approximations are guided by the choice of wave function and the partitioning of the variational form. In this paper the method is applied to the long range interactions of helium, and in more approximate forms to HHe and LiHe. For He 2, studies are made of two- and three-electron excitations. Three-electron excitations contribute almost 9% to the value of the C 6 coefficient. Our final C 6 coefficient is 1.457 (in atomic units) versus the best semi-empirical value of 1.4618 ≠ 0.0004. The results are compared with other calculations in a formal way so as to demonstrate the value of our alternative expressions.
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