1. A computer-implemented method, comprising: receiving a system model having a first system order, wherein the system model contains a plurality of system nodes, a plurality of system matrices; determining an expansion frequency of the system; factorizing the plurality of system matrices at the expansion frequency by a matrix decomposition; constructing a plurality of reduced order system matrices without calculating a projection matrix; and constructing a reduced order system model having a second system order with the plurality of reduced order system matrices, wherein the second system order is lower than the first system order.