...
Answer: For dynamic analysis, Ritz vectors are recommended over Eigen vectors because, for the same number of modes, Ritz vectors provide a better participation factor, which enables the analysis to run faster with the same level of accuracy. When analysis involves ground motion in the horizontal plane, the benefit is not as pronounced. However, for vertical acceleration, localized machine vibration, and the nonlinear FNA method, Ritz vectors are much more well-suited for analysis.
...