frsblk_wrapper Subroutine

    subroutine frsblk_wrapper(det_list, ndets, nexcit, evals, evecs)

        ! A wrapper to the frsblk Lanczos routine. This wrapper has a much
        ! smaller number of input parameters than frsblkh. The input parameters
        ! are:

        ! det_list: A list of all determinants in the space to be diagonalised,
        !     in orbital form, i.e. det_list(:,i) contains the spin orbitals
        !     occupied in the i'th determinant.
        ! ndets: The number of determinants in the space to be diagonalised,
        !     which should be equal to the size(det_list,2).
        ! nexcit: The number of eigenvectors to calculate. The nexcit lowest
        !     states will be calculated.

        ! On input, evals and evecs should be allocated already.

        ! On output, evals stores the nexict lowest eigenvalues and evecs
        ! stores the corresponding eigenvectors.

        use DetCalcData, only: nkry, nblk, b2l, ncycle
        use sort_mod, only: sort
        use SystemData, only: nel, tHPHF, tGUGA

        integer, intent(in) :: det_list(:, :)
        integer, intent(in) :: ndets
        integer, intent(in) :: nexcit
        real(dp), intent(out) :: evals(:)
        real(dp), intent(out) :: evecs(:, :)

        integer :: ierr, nkry1, nblock, len_scr, len_iscr, ICMax, LenHamil, i
        integer, allocatable :: nRow(:), Lab(:), iscr(:), ind(:)
        real(dp), allocatable :: evecs_space(:, :), Hamil(:), A_Arr(:, :)
        real(dp), allocatable :: V(:), BM(:), T(:), WT(:), scr(:), AM(:)
        logical :: tMC
        character(len=*), parameter :: t_r = 'frsblk_wrapper'

#ifdef CMPLX_
        call stop_all(t_r, 'frsblk cannot work with complex wavefunctions currently')
#endif

        allocate(nRow(ndets), stat=ierr)
        nRow = 0
        ICMax = 1
        tMC = .false.

        ! just to make sure we pass valid objects
        allocate(Lab(1), stat=ierr)
        allocate(Hamil(1), stat=ierr)
        call Detham(ndets, nel, det_list, Hamil, Lab, nRow, .true., ICMax, LenHamil, tMC)

        deallocate(Hamil, stat=ierr)
        deallocate(Lab, stat=ierr)

        allocate(Hamil(LenHamil), stat=ierr)
        if (ierr /= 0) call stop_all(t_r, "Error allocating Hamil.")
        allocate(Lab(LenHamil), stat=ierr)
        if (ierr /= 0) call stop_all(t_r, "Error allocating Lab.")

        Hamil = 0.0_dp
        Lab = 0

        call Detham(ndets, NEl, det_list, Hamil, Lab, nRow, .false., ICMax, LenHamil, tMC)

        nkry1 = nkry + 1
        nblock = min(nexcit, nblk)
        len_scr = max(ndets * nexcit, 8 * nblock * nkry)
        len_iscr = 6 * nblock * nkry

        allocate(A_Arr(nexcit, nexcit), stat=ierr)
        allocate(V(ndets * nblock * nkry1), stat=ierr)
        allocate(AM(nblock * nblock * nkry1), stat=ierr)
        allocate(BM(nblock * nblock * nkry), stat=ierr)
        allocate(T(3 * nblock * nkry * nblock * nkry), stat=ierr)
        allocate(WT(nblock * nkry), stat=ierr)
        allocate(scr(len_scr), stat=ierr)
        allocate(iscr(len_iscr), stat=ierr)
        allocate(ind(nexcit), stat=ierr)
        allocate(evecs_space(ndets, nexcit), stat=ierr)

        A_Arr = 0.0_dp
        V = 0.0_dp
        AM = 0.0_dp
        BM = 0.0_dp
        T = 0.0_dp
        WT = 0.0_dp
        scr = 0.0_dp
        iscr(1:len_iscr) = 0
        ind(1:nexcit) = 0
        evecs_space = 0.0_dp

        ! Perform Lanczos procedure.
        call neci_frsblkh(ndets, ICMax, nexcit, Hamil, Lab, evecs, evecs_space, nkry, nkry1, nblock, nrow, &
                          len_scr, len_iscr, A_Arr, evals, V, AM, BM, T, WT, scr, iscr, ind, ncycle, b2l, &
                          .false., .false., .true.)

        ! The above routine returns *minus* the eigenvalues. Remove this factor:
        evals = -evals

        ! Sometimes eigenvalues and eigenvectors aren't returned in the correct
        ! order, so reorder them so that the lowest-energy ones are first.
        ! If they are degenerate, then we still need to use a fixed ordering,
        ! so that different compilers are consistent.
        call sort(evals, evecs)

        deallocate (evecs_space, &
                    A_Arr, &
                    V, &
                    BM, &
                    T, &
                    WT, &
                    scr, &
                    iscr, &
                    ind, &
                    AM, &
                    nrow, &
                    Lab, &
                    Hamil)

    end subroutine frsblk_wrapper