Before revision:  On the other hand, there are very challenging tasks to find reduced-order form of a large-scale system with a minimized \mathcal{H}_2 error norm and attain the rapid rate of convergence.
After revision:  On the other hand, there are very challenging tasks to find reduced-order form of a large-scale second-order systems with the optimal \mathcal{H}_2 error norm and attain the rapid rate of convergence.