Both the FFEPlus and Direct Sparse solvers detect rigid body modes automatically for unrestrained or not adequately supported models. Rigid body modes, if detected, are counted among the requested number of modes.
For example, if you are interested in the first five elastic modes for an unsupported solid model, you need to set the required modes to 11: six for the rigid body modes and five for the elastic modes.
If you use the Direct Sparse solver, you must apply adequate restraints to stabilize your model, or activate the Use soft spring to stabilize model option. However, note that improper application of restraints can over-stiffen the model, and adversely affect the elastic modes.
In cases where the frequency analysis cannot run due to singularity of the stiffness matrix, use the frequency shift option to overcome the singularity issue. Increase the shift value gradually from zero until the Direct Sparse solver successfully calculates the requested frequencies.
If you set a higher value for the frequency shift, the Direct Sparse solver selectively calculates the requested number of frequencies, which are clustered around the shift value. Thus, you can avoid the computation of lower range frequencies (including the rigid body modes), which are of no interest to your analysis, and save computational time.