get_helement_excit Function

    function get_helement_excit(NI, NJ, IC, ExcitMat, TParity) result(hel)

        ! Calculate the Hamiltonian Matrix Element for a given determinant (or
        ! csf), when we have the excitation matrix and parity of the
        ! excitation.
        !
        ! In:  nI, nJ       - The determinants to evaluate
        !      IC           - The number of orbitals I,J differ by
        !      ex           - The excitation matrix
        !      tParity      - The parity of the excitation
        ! Ret: hel          - The H matrix element

        integer, intent(in) :: nI(nel), nJ(nel), IC
        integer, intent(in) :: ExcitMat(2, ic)
        logical, intent(in) :: tParity
        HElement_t(dp) :: hel

        character(*), parameter :: this_routine = 'get_helement_excit'

        ! intermediately put the special call to the hubbard matrix elements
        ! here. Although I want to change that in the whole code to have
        ! procedure pointers similar to the excitation generator, which gets
        ! intialized to the correct function at the beginning of the
        ! excitations
        ! store all the lattice model matrix elements in one call.
        if (t_lattice_model) then
            hel = get_helement_lattice(nI, ic, ExcitMat, tParity)
            return
        end if

        ! GUGA implementation:
        if (tGUGA) then
            if (all(nI == nJ)) then
                hel = calcDiagMatEleGUGA_nI(nI)
                return
            end if

        end if

        if (IC < 0) then
            call stop_all(this_routine, "get_helement_excit should only be &
                         &used if we know the number of excitations and the &
                         &excitation matrix")
        end if

        hel = dyn_sltcnd_excit_old(nI, IC, ExcitMat, tParity)

        if (IC == 0) then
            hel = hel + (ECore)
        else if (tStoquastize) then
            hel = -abs(hel)
        end if

    end function get_helement_excit