But
===

This command makes it possible to **calculate the real or complex natural modes,** as well as **the harmonic response** (displacement, speed, acceleration) of a **structure comprising viscoelastic materials whose properties depend on the frequency** (which results in a stiffness matrix dependent on the frequency).

The following actions are possible:


* obtaining modes by iterative method; 3 types of modes to choose from: real modes, real beta-modes or complex modes, taking into account the frequency dependence of the mechanical properties of viscoelastic materials;


* if there is an excitation, calculation of the harmonic response of the structure; the calculation is done by projection on the modal base established in the previous step, then reproduction on the physical base.

For more theoretical details, refer to the reference document [:ref:`R5.05.10 <R5.05.10>`].

The command can produce a concept such as mode_meca, mode_meca_c, dyna_harmo.

**Note:*

*The command does not currently allow the calculation of dual modes;*

*The calculation of structures with rigid body modes may not converge.*