perform_davidson Subroutine

    subroutine perform_davidson(this, hamil_type_in, print_info_in)
        use mpi
        integer, intent(in) :: hamil_type_in
        logical, intent(in) :: print_info_in
        logical :: print_info
        integer :: i
        real(dp) :: start_time, end_time
        type(DavidsonCalcType), intent(inout) :: this
        character(*), parameter :: this_routine = "perform_davidson"

        ! Only let the root processor print information.
        print_info = print_info_in .and. (iProcIndex == root)

        call InitDavidsonCalc(this, print_info, hamil_type_in)

        if (print_info) write(stdout, '(1X,"Iteration",4X,"Residual norm",12X,"Energy",7X,"Time")'); call neci_flush(stdout)

        do i = 2, max_num_davidson_iters

            if (this%super%skip_calc) exit

            start_time = MPI_WTIME()

            if (iProcIndex == root) call subspace_expansion(this, i)

            call project_hamiltonian(this, i)

            if (iProcIndex == root) call subspace_extraction(this, i)

            call calculate_residual(this, i)

            call calculate_residual_norm(this)

            end_time = MPI_WTIME()

            if (print_info) write(stdout, '(8X,i2,3X,f14.9,2x,f16.10,2x,f9.3)') i - 1, this%residual_norm, &
                this%davidson_eigenvalue, end_time - start_time; call neci_flush(stdout)

            if (this%residual_norm < residual_norm_target) exit

            if (i == max_num_davidson_iters) then
                call stop_all(this_routine, "Davidson iteration reached the maximum number of iterations. &
                                            &The deterministic energy may not be converged. &
                                            &You can increase 'davidson-max-iters' or 'davidson-target-tolerance'.")
            end if

        end do

        if (print_info) write(stdout, '(/,1x,"Final calculated correlation energy:",1X,f16.10)') this%davidson_eigenvalue

        call FreeDavidsonCalc(this)

    end subroutine perform_davidson