Starting from a molecular Hamiltonian : H ^ ^ tot = ∑ ∑ i p i 2 2 m + ∑ ∑ I P I 2 2 M I + ∑ ∑ i V nucl ( r i ) + 1 2 ∑ ∑ i ≠ ≠ j e 2 | r i − − r j | + 1 2 ∑ ∑ I ≠ ≠ J Z I Z J e 2 | R I − − R J | where r i are the electrons and R I are the nuclei and