calcB_vector_ilut Function

    pure function calcB_vector_ilut(ilut) result(bVector)
        ! function to calculate bVector of length (nBasis) for a given
        ! CSF bitRep ilut of length (2*nBasis), save this b-vector
        ! in the nI array, normally used to store occupied orbitals
        ! update: changed convention to not use nI for bVector but also for
        ! occupied orbitals as in normal NECI implementation
        ! but nethertheless need a b-vector of lenght of spatial orbitals
        ! for excitaiton and matrix element calculation
        ! UPDATE: change b vector calculation to update b vector directly on
        ! the spot of the corresponding stepvector, eg:
        ! d = 0 1 2 3
        ! b = 0 1 0 0
        ! because otherwise i actually only needed the bvector of one orbital
        ! ahead. and never the first entry otherwise. and that would cause
        ! problems when accessing b vector at the end of an excitaiton if
        ! that is the last orbital..
        integer(n_int), intent(in) :: ilut(0:GugaBits%len_orb)
        real(dp) :: bVector(nSpatOrbs)        ! b-vector stored in bVector
        integer :: i
        real(dp) :: bValue

        ! loop over CSF entries and increment, decrement bValue
        ! accordingly, have to correctly access ilut entries and
        ! check if they are 0,1,2 or 3... -> how to for multiple
        ! integers?
        bVector = 0.0_dp
        bValue = 0.0_dp

        do i = 1, nSpatOrbs

            if (isOne(ilut, i)) then
                bValue = bValue + 1.0_dp
            else if (isTwo(ilut, i)) then
                bValue = bValue - 1.0_dp
            end if
            ! define bvalue to always only get updated for the next
            ! UPDATE: changed definition to update on the spot.
            bVector(i) = bValue
        end do

    end function calcB_vector_ilut