getExcitationRangeMask Function

    function getExcitationRangeMask(i, j) result(mask)
        ! function to create an integer corresponding to a mask were every
        ! bits between the spin orbitals corresponding to spatial orbitals
        ! i and j are set to 1 and 0 else. used to access the excitation
        ! range for GUGA excitation calculations
        integer, intent(in) :: i, j
        integer(n_int) :: mask
        character(*), parameter :: this_routine = "getExcitationRangeMask"

        integer(n_int) :: k
        integer(n_int) :: tmp_i, tmp_j

        ASSERT(i > 0 .and. i <= nSpatOrbs)
        ASSERT(j > 0 .and. j <= nSpatOrbs)
        ASSERT(j > i)

        tmp_i = int(i, n_int)
        tmp_j = int(j, n_int)

        ! not quite sure about LMSB or RMSB... todo
        mask = 0_n_int

        do k = 2_n_int * tmp_i - 1_n_int, 2_n_int * tmp_j ! convert to spin orbitals
            mask = mask + 2_n_int**(k - 1_n_int)
        end do

    end function getExcitationRangeMask