# The model description and basic model bricks¶

The model description of GetFEM allows to quickly build some fem applications on complex linear or nonlinear PDE coupled models. The principle is to propose predefined bricks which can be assembled to describe a complex situation. A brick can describe either an equation (Poisson equation, linear elasticity …) or a boundary condition (Dirichlet, Neumann …) or any relation between two variables. Once a brick is written, it is possible to use it in very different situations. This allows a reusability of the produced code and the possibility of a growing library of bricks. An effort has been made in order to facilitate as much as possible the definition of a new brick. A brick is mainly defined by its contribution in the tangent linear system to be solved.

This model description is an evolution of the model bricks of previous versions of GetFEM. Compared to the old system, it is more flexible, more general, allows the coupling of model (multiphysics) in a easier way and facilitates the writing of new components. It also facilitate the write of time integration schemes for evolving PDEs.

The kernel of the model description is contained in the file getfem/getfem_models.h. The two main objects are the model and the brick.