The Lanczos eigenvalue solver calculates exact natural models (i.e., eigenvectors) and exact and frequencies. It is much faster than other two solutions (i.e., the subspace and Ritz vectors) and it consumes significantly less memory. For this reason, this solution is recommended for large models.

It should be noted that both the Lanczos solution and subspace solution produce the same results for well-conditioned systems. The implementation is an open source code developed by Rice University. See References section for more information.