Localized orbitals and short-range molecular interactions. II. Nonadditivity in He3 and He4

Short-range interatomic interactions in singlet He3 and He4 are discussed in terms of a single determinant of doubly occupied nonorthogonal localized orbitals through the formalism described in Paper I [V. Magnasco and G. F. Musso, J. Chem. Phys. 60, 3744 (1974)]. Ab-initio calculations using a minimal set of Slater 1s orbitals are reported for several nuclear configurations. Nonadditive terms can be very well represented as resulting from penetration of pairs of nonorthogonal localized electron groups into the core provided by nuclei and ``unperturbed'' electrons of all other groups. Even in the small overlap region it is essential to include all the penetration effects, and this is easily done by the present formalism.