where α = c -1 is the fine structure constant with c speed of light, liI = (r i -R I ) × pi is the angular momentum of the i-th electron relative to the I-th nucleus at position R I with charge Z I , and li j = (r ir j ) × pi the angular momentum of the i-th electron relative to the j-th electron, with ri and pi position and momentum vector operators. Finally, ŝi is the spin operator of the i-th electron.