The purpose of this page is to explain the warning message "Eigenvalue was found out of sequence".
Modes are found in order of increasing distance of frequency from the shift. If lower frequency is found after a larger frequency, the program display the "out of sequence" warning. Please note that this is a warning, not an error, and may occur if the structure is sensitive to instabilities.
A structure that is unstable when unloaded will have some modes with zero frequency. These modes may correspond to rigid- body motion of an inadequately supported structure, or to mechanisms that may be present within the structure. It is not possible to compute the static response of such a structure. However, by using a small negative shift, the lowest- frequency vibration modes of the structure, including the zero- frequency in stability modes, can be found.