multipole_expand_scalar_product(a, b, P, Q, c, d)
Multipole-expand the matrix element
⟨ab|P⋅Q|cd⟩, where the tensor
P acts on orbitals
c, and the tensor
Q acts on orbitals
d. The definition is taken from Eq. (13.1.26) of Varshalovich (1988).
Multipole-expand the two-body integral resulting from the Coulomb repulsion between two electrons.