init_exact_diag Subroutine

    subroutine init_exact_diag()

        use bit_rep_data, only: NIfTot
        use gndts_mod, only: gndts_all_sym_this_proc
        use SystemData, only: nbasis, nel
        use util_mod, only: choose_i64

        integer :: expected_ndets_tot
        integer(n_int), allocatable :: ilut_list(:, :)
        character(len=*), parameter :: t_r = 'init_exact_diag'
        integer :: ierr

        write(stdout, '(/,1x,a57,/)') "Beginning exact diagonalisation in all symmetry sectors."
        call neci_flush(stdout)

        write(stdout, '(1x,a56)', advance='no') "Enumerating and storing all determinants in the space..."
        call neci_flush(stdout)

        ! Generate and count all the determinants on this processor, but don't store them.
        call gndts_all_sym_this_proc(ilut_list, .true., ndets_ed)
        allocate(ilut_list(0:NIfTot, ndets_ed))
        ! Now generate them again and store them this time.
        call gndts_all_sym_this_proc(ilut_list, .false., ndets_ed)

        write(stdout, '(1x,a9)') "Complete."
        call neci_flush(stdout)

        expected_ndets_tot = int(choose_i64(nbasis, nel))
        if (ndets_ed /= expected_ndets_tot) then
            write(stdout, *) "ndets counted:", ndets_ed, "ndets expected:", expected_ndets_tot
            call stop_all('t_r', 'The number of determinants generated is not &
                                    &consistent with the expected number.')
        end if

        allocate(eigv_ed(ndets_ed), stat=ierr)
        if (ierr /= 0) then
            write(stdout, '(1x,a11,1x,i5)') "Error code:", ierr
            call stop_all(t_r, "Error allocating eigenvalue array.")
        end if
        eigv_ed = 0.0_dp

        write(stdout, '(1x,a48)') "Allocating and calculating Hamiltonian matrix..."
        call neci_flush(stdout)
        allocate(hamiltonian(ndets_ed, ndets_ed), stat=ierr)
        if (ierr /= 0) then
            write(stdout, '(1x,a11,1x,i5)') "Error code:", ierr
            call stop_all(t_r, "Error allocating Hamiltonian array.")
        end if
        write(stdout, '(1x,a46)') "Hamiltonian allocation completed successfully."
        call neci_flush(stdout)
        call calculate_full_hamiltonian(ilut_list, hamiltonian)
        write(stdout, '(1x,a33)') "Hamiltonian calculation complete."
        call neci_flush(stdout)

    end subroutine init_exact_diag