check_delta Function

    function check_delta(this, k, i_state) result(t)
        type(LanczosCalcType), intent(inout) :: this
        integer :: k, i_state
        logical :: t
        if (k <= this%n_states) then
            t = .false.
            return
        end if
        t = (abs(this%ritz_values(i_state) - this%ritz_values_old(i_state)) < this%convergence_error)
    end function check_delta