(M)RHD numerical simulations: powerful tool for insight into (variable) stars

The ever-increasing inflow and steadily-improving quality of observational data of (variable) stars needs to stand on the solid footing of theory and relevant numerical simulations. Derivation of stellar parameters follows from comparison with models. Only the use of the best achievable tools, namely (magneto)-radiation-hydrodynamics -- (M)RHD -- stellar atmospheric models coupled with advanced radiative transfer, ensures optimal results. However, these advanced models can only be obtained via time-consuming supercomputer simulations. Therefore, a compromised approach makes sense to achieve the speed and efficiency in properly analysing the vast amount of observational data. The presentation includes a discussion on current knowledge about the Sun, terrestrial climate, stellar magnetic cycles, and numerical modelling of stellar surfaces.