add_en2_pert_for_init_calc Subroutine

    subroutine add_en2_pert_for_init_calc(ispawn, abort, nJ, SpawnedSign)

        ! Add a contribution to the second-order Epstein-Nesbet correction to
        ! initiator error.

        ! This adds a correction for the determinant at position ispawn in
        ! the spawning array, and nJ is an array holding the occupied
        ! electrons on this determinant. SpawnedSign is the array of
        ! amplitudes spawned onto this determinant, and abort is array
        ! specifying whether each of the spawnings are about to be aborted
        ! due to the initiator criterion.

        integer, intent(in) :: ispawn
        logical, intent(in) :: abort(lenof_sign)
        integer, intent(in) :: nJ(nel)
        real(dp), intent(in):: SpawnedSign(lenof_sign)

        integer :: istate
        real(dp) :: contrib_sign(en_pert_main%sign_length)
        logical :: pert_contrib(en_pert_main%sign_length)

        ! RDM-energy-based estimate:
        ! Only add a contribution if we've started accumulating this estimate.
        if (tEN2Started) then
            pert_contrib = .false.

            do istate = 1, en_pert_main%sign_length
                ! Was a non-zero contribution aborted on *both* replicas for
                ! a given state?
                if (abort(2 * istate - 1) .and. abort(2 * istate) .and. &
                    abs(SpawnedSign(2 * istate - 1)) > 1.e-12_dp .and. &
                    abs(SpawnedSign(2 * istate)) > 1.e-12_dp) &
                    pert_contrib(istate) = .true.
            end do

            if (any(pert_contrib)) then
                contrib_sign = 0.0_dp
                do istate = 1, en_pert_main%sign_length
                    if (pert_contrib(istate)) then
                        contrib_sign(istate) = SpawnedSign(2 * istate - 1) &
                            * SpawnedSign(2 * istate) / (tau**2)
                    end if
                end do

                call add_to_en_pert_t(en_pert_main, nJ, SpawnedParts(:, ispawn), contrib_sign)
            end if
        end if

    end subroutine add_en2_pert_for_init_calc