  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. Here, we model with a high degree of precision
the optical and mechanical modes that are involved in these parametric
instabilities, such that our model can become predictive. As an example, 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 with unprecedented level of detail and
accuracy. 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.
