pick_virtual_electron_single Subroutine

    subroutine pick_virtual_electron_single(nI, part_type, elec, pgen_elec, calc_pgen)
        ! same as above for a single excitation
        ! remember: elec is really just the number in the ilut!
        integer, intent(in) :: nI(nel), part_type
        integer, intent(out) :: elec
        real(dp), intent(out) :: pgen_elec
        logical, intent(in), optional :: calc_pgen

        character(*), parameter :: this_routine = "pick_virtual_electron_single"

        integer :: i, n_valid, j, ind
        integer:: virt_elecs(nel)

        ! what do we need here?
        ! count all the electrons in the virtual of the reference, then
        ! create a list of them and pick one uniformly
        n_valid = 0
        j = 1
        virt_elecs = -1
        elec = -1

        do i = 1, nel
            if (is_in_virt_mask(nI(i), part_type)) then
                ! the electron is in the virtual of the
                n_valid = n_valid + 1
                virt_elecs(j) = i
                j = j + 1
            end if
        end do

        if (n_valid == 0) then
            ! something went wrong
            ! yes.. this means it was called on the hartree fock..
            ! but i think aborting the excitation is enough.. i do not
            ! need to stop_all or? but this could catch some bugs..
            elec = 0
            pgen_elec = 0.0_dp
            return
        end if

        ! does this work with an optional logical as input:
        ASSERT(n_valid > 0)

        pgen_elec = 1.0_dp / real(n_valid, dp)

        ! if i only want to calculate the pgens i dont want to call the
        ! random number generator
        if (present(calc_pgen)) then
            if (calc_pgen) return
        end if

        ! and now pick a random number:
        ind = 1 + floor(genrand_real2_dSFMT() * n_valid)
        elec = virt_elecs(ind)
        ASSERT(elec > 0)

    end subroutine pick_virtual_electron_single