guess_target_nref Function

    function guess_target_nref() result(target_nref)
        ! try to extrapolate how many SIs are required for a given target population
        use FciMCData, only: AllNoatHF
        implicit none
        integer :: target_nref
        real(dp) :: alpha, beta, cAllNoatHF

        target_nref = nRefs
        cAllNoatHF = sum(AllNoatHF) / inum_runs
        ! if the reference population did not change, nothing to do
        if (lastAllNoatHF.isclose.cAllNoatHF) return

        ! if we did not change the SI threshold, nothing to do
        if (lastNRefs == nRefs) return
        beta = log(lastAllNoatHF / cAllNoatHF) / (lastNRefs - nRefs)
        alpha = lastAllNoatHF * exp(log(lastAllNoatHF / cAllNoatHF) * (lastNRefs / (lastNRefs - nRefs)))

        ! here, we really assume exponential behaviour and extrapolate the nRefs that would then
        ! give the reference population closest to the target
        target_nref = int(log(targetRefPop / alpha) / beta)

    end function guess_target_nref