GAS_doubles_PCHB_spinorb_get_pgen Function

    real(dp) function GAS_doubles_PCHB_spinorb_get_pgen(this, nI, ilutI, ex, ic, ClassCount2, ClassCountUnocc2) result(pgen)
        !! calculates the probability of drawing a given double excitation
        !! parametrised by the excitation matrix ex
        class(GAS_PCHB_DoublesSpinOrbFastWeightedExcGenerator_t), intent(inout) :: this
        integer, intent(in) :: nI(nel)
        integer(n_int), intent(in) :: ilutI(0:NIfTot)
        integer, intent(in) :: ex(2, maxExcit), ic
            !! excitation matrix
        integer, intent(in) :: ClassCount2(ScratchSize), ClassCountUnocc2(ScratchSize)
        integer :: i_sg, IJ, AB
        character(*), parameter :: this_routine = 'GAS_doubles_PCHB_spinorb_get_pgen'

#ifdef WARNING_WORKAROUND_
        associate(ilutI => ilutI); end associate
        associate(ClassCount2 => ClassCount2); end associate
        associate(ClassCountUnocc2 => ClassCountUnocc2); end associate
#endif
#ifdef DEBUG_
    block
        use util_mod, only: stop_all
        use constants, only: stderr
        if (.not. (ic == 2)) then
            write(stderr, *) ""
            write(stderr, *) "Assertion ic == 2"
            write(stderr, *) "failed in /scratch/jenkins/jobs/existing_branch_doc/workspace/build_config/gfortran-doc/src/gasci_pch&
                &b_doubles_spinorb_fastweighted.fpp:259"
            call stop_all (this_routine, "Assert fail: ic == 2")
        end if
    end block
#endif

        IJ = fuseIndex(ex(1, 1), ex(1, 2))
        AB = fuseIndex(ex(2, 1), ex(2, 2))
        i_sg = this%indexer%idx_nI(nI)

        pgen = this%particle_selector%get_pgen(nI, i_sg, ex(1, 1), ex(1, 2))
        pgen = pgen * this%AB_sampler%get_prob(IJ, i_sg, AB)

    end function GAS_doubles_PCHB_spinorb_get_pgen