Spin and charge densities from the Hiller—Sucher—Feinberg identity: double-perturbation calculations for BeH

Abstract

Møller—Plesset (MP) and Epstein—Nesbet (EN) types of double-perturbation theory are used for the calculation of the spin and charge densities at the nuclei of the ground state of BeH with the Hiller—Sucher—Feinberg (HSF) identity. Within sigle- and double-electron excitations for the configuration state functions, it is not sufficient to take the second-order contribution with respect to the electron correlation into account; indeed, for a good spin density it is necessary to take the fourth-order contribution into account, in each MP (SD-MP) or EN (SD-EN). The fourth-order MP (or EN) value of 0.309 (or 0.306) bohr −3 for the HSF spin density at the Be nucleus is in excellent agreement with the experimental value of 0.317 bohr −3. For the HSF spin density at H, the fourth-order MP (or EN) value of 0.0421 (or 0.0437) bohr −3 is also in excellent agreement with the experimental value of 0.0434 bohr −3. It can be shown that the HSF identity is an excellent tool for calculating spin and charge densities accurately, not only in configuration—interaction theory but also in double-perturbation theory.