calculate_residual_norm_ss Subroutine

    subroutine calculate_residual_norm_ss(this)

        type(davidson_ss), intent(inout) :: this

        real(dp) :: residual_norm_tot

        ! This subroutine calculates the Euclidean norm of the reisudal vector, r:
        ! residual_norm^2 = \sum_i r_i^2
        if (this%space_size_this_proc > 0) then
            this%residual_norm = dot_product(this%residual, this%residual)
        else
            this%residual_norm = 0.0_dp
        end if

        call MPISumAll(this%residual_norm, residual_norm_tot)

        this%residual_norm = sqrt(residual_norm_tot)

    end subroutine calculate_residual_norm_ss