Why Ritz vectors are strongly recommended for dynamic analysis?
For the same number of modes, Ritz vectors give you a better participation factor than do Eigenvectors. Therefore, the analysis should run faster to get the same accuracy using Ritz rather than Eigenvectors. For certain types of loading, such as vertical acceleration, localized machine vibration, and the nonlinear FNA method, Ritz vectors are much better than Eigenvectors. For horizontal ground acceleration on typical building structures, it is not so critical.