With this option you can ask for eigenvalues (and eigenvectors). The eigenvectors are determined for the principal unknowns in the system (like velocity, temperature, etc.). This option is only available after the matrices have been set up with control_options_solver. And it is assumed that the matrices used in the eigenmodes determination are symmetric.
With type_of_eigen_analysis you can determine which eigen problem will be solved. If type_of_eigen_analysis is set to -matrix, the eigen modes of the matrix will be solved. Other options for type_of_eigen_analysis are presently not available. (To be more clear: presently you cannot solve generalized eigenvalue problems like vibration in mechanics, acoustics, etc.)
The eigenvalues are put in the control_eigen_values record. The eigenvectors are put in the node_eigen records.
Warning: the eigensolvers we use are not tuned for finite element analysis. So the eigensolvers may be very slow, and large amounts of memory may be consumed.
The eigenvalues and eigenvectors are deleted whenever a new timestep is performed with control_timesteps. This is done since in general after a new timestep the matrices will have changed, and so the eigenvalues are not valid anymore.
See also control_eigen_scale.