A calculation of the helium pair polarizability including correlation effects

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Abstract
Direct configuration interaction (CI) calculations, which include all single and double excitation functions, and the finite-field method have been used to calculate the polarizability of a pair of helium atoms at a number of separations in order to assess the importance of correlation effects. A limited basis set of gaussian expansions of Slater orbitals was used. The basis set located on each atom was 5s, 3p, 2d, and was chosen to provide reasonably accurate atomic wavefunctions for both zero and finite electric fields. Agreement is obtained with the long-range results of perturbation theory. For incremental and incremental mean polarizabilities it is apparent that calculation of both self-consistent field (SCF) and CI contributions is required for an accurate determination. The anisotropy, however, seems to be calculable directly from an SCF calculation provided a simple scaling procedure is adopted. Counterpoise corrections have been calculated and found to be appreciable, especially for the incremental mean polarizability at long range.