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[FAQ]

There is no guarantee or restriction to require that the rigid-body modes are parallel to the global Cartesian coordinate system. Consequently, if the calculated rigid-body modes aren’t aligned with global Cartesian, they may look strange. What matters is only that the rigid-body modes (eigenvectors) are orthogonal to each other. Hence, there should be a linear combination of modes 1 through 6 to equate rigid-body translation along the global X direction, for example. Rigid-body motion has no unique solution. The basis taken is not orthogonal to the global CS doesn’t mean that it’s incorrect.