For the calculation of structural periods and modes, there are three methods provided in the program: Subspace iteration, Arnoldi/Lanczos eigenvalue solution, and (Load Dependent) Ritz vectors.

The subspace iteration solution is the default choice in the program. This solution calculates natural modes (i.e., eigenvectors) and frequencies of an undamped free vibration system. These natural modes and frequencies are the exact modes and frequencies and they provide excellent information about dynamic characteristics of the system.

The Arnoldi/Lanczos eigenvalue solution also calculates exact natural models (i.e., eigenvectors) and frequencies and it is much faster than other two solutions and consumes significantly less memory. For large models, this solution is recommended.

It should be noted that both the Lanczos solution and Subspace solution produce the same results.

Solution with the Ritz Vectors is an approximate solution to eigenvectors but it produces results that are the same or nearly the same as the eigenvectors if enough number of Ritz vectors are included. For the same number of modes, Ritz vectors usually provide a better mass participation and it is computationally faster with almost the same level of accuracy. The Ritz vectors method is recommended where the solution with eigenvectors may not reliably capture lateral structural models (for instance, failing to capture 90 percent mass participation with a reasonable number of modes) or the eigenvectors solution captures irrelevant vertical models due to vertical vibration of slabs (although those are real modes but they are not relevant to the structural analysis). The method of Ritz vectors also has the advantage of being generally faster. For this reason, use of Ritz vectors is also an attractive for very large models.