cepa_acpf Function

    real(dp) function cepa_acpf(run)
        integer, intent(in) :: run
        ! do i use the shift or the projected energy here?? tbd
        ! change the implementation, so that it actually gives
        ! S - D = 2S/N -> D = S(1 - 2/N)
        cepa_acpf = diagsft(run) * (1.0_dp - 2.0_dp / real(nel, dp))

    end function cepa_acpf