contract_1_rdm_ind Function

    pure function contract_1_rdm_ind(i, a, excit_lvl, excit_typ) result(rdm_ind)
        ! the inverse function of the routine above, to give the combined
        ! rdm index of two explicit ones
        integer, intent(in) :: i, a
        integer, intent(in), optional :: excit_lvl, excit_typ
        integer(int_rdm) :: rdm_ind

        rdm_ind = nSpatOrbs * (i - 1) + a

        if (present(excit_lvl)) then
            call encode_excit_lvl_rdm(rdm_ind, excit_lvl)
        end if

        if (present(excit_typ)) then
            call encode_excit_typ_rdm(rdm_ind, excit_typ)
        end if

    end function contract_1_rdm_ind