calc_pgen_triple Function

    function calc_pgen_triple(nI, ex) result(pgen)
        ! get the probability to get excitation `ex` from a determinant `nI`
        integer, intent(in) :: nI(nel)
        integer, intent(in) :: ex(2, 3)
        real(dp) :: pgen
        integer :: ms, i, tgt_spin
        character(*), parameter :: t_r = "calc_pgen_triple"

        ! get the spin
        ms = 0
        ! sum up the spin of the single orbitals
        do i = 1, 3
            ms = ms + G1(ex(2, i))%ms
        end do

        ! start with pTriples
        pgen = pTriples
        ! then, add the spin bias and electron picking probs
        if (ms == -3) then
            pgen = pgen * p0A * pgen3B
        else if (ms == -1) then
            pgen = pgen * p2B * pgen2B
        else if (ms == 1) then
            pgen = pgen * (1 - p2B - p0A - p0B) * pgen1B
        else if (ms == 3) then
            pgen = pgen * p0B * pgen0B
        else
            call stop_all(t_r, "Invalid spin")
        end if

        ! Add the probability of picking these three target orbitals
        if (tNoSymGenRandExcits) then
            call calc_pgen_triple_target_nosym(ms, pgen)
        else
            call calc_pgen_triple_target_sym(nI, ex, ms, pgen)
        end if
    end function calc_pgen_triple