get_hdiag_from_sing_excit Function

    function get_hdiag_from_sing_excit(nI, ex, hel_old) result(hel)

        ! Calculate the  by the SlaterCondon Rules when the two
        ! determinants are the same (so we only need to specify one).

        integer, intent(in) :: nI(nel), ex(2)
        real(dp), intent(in) :: hel_old

        HElement_t(dp) :: hel
        integer :: id(nel), id_ex(2), i

        ! Correct one electron integral contribution
        hel = hel_old - GetTMATEl(ex(1), ex(1)) + GetTMATEl(ex(2), ex(2))

        ! Obtain the spatial rather than spin indices if required
        id = gtID(nI)
        id_ex = gtID(ex)

        do i = 1, nel
            if (nI(i) /= ex(1)) then
                hel = hel - get_umat_el(id_ex(1), id(i), id_ex(1), id(i)) &
                      + get_umat_el(id_ex(2), id(i), id_ex(2), id(i))

                if (tReltvy .or. (G1(ex(1))%Ms == G1(nI(i))%Ms)) then
                    hel = hel + get_umat_el(id_ex(1), id(i), id(i), id_ex(1)) &
                          - get_umat_el(id_ex(2), id(i), id(i), id_ex(2))
                end if
            end if
        end do

    end function get_hdiag_from_sing_excit