  Increasing the laser power is essential to improve the sensitivity of
interferometric gravitational wave detectors. However, optomechanical
parametric instabilities can set a limit to that power. It is of major
importance to understand and characterize the many parameters and effects that
influence these instabilities. We perform simulations for the Advanced Virgo
interferometer (O3 configuration). In particular we compute mechanical modes
losses by combining both on-site measurements and finite element analysis. We
also study the influence on optical modes and parametric gains of mirror finite
size effects, and mirror deformations due to thermal absorption. We show that
these effects play an important role if transverse optical modes of order
higher than four are involved in the instability process.
