The use of the framework flexibility in molecular dynamics (MD) computer simulation of zeolites and related materials is discussed in detail and from different perspectives. Once ascertained that a flexible framework is needed to ensure, in the micro-canonical ensemble, the exact, in principle, conservation of linear momentum of the simulated system, the practical effects of keeping the framework fixed on several simulated quantities and processes are described. In particular, the diffusivity, the activated processes, the framework deformations and the approximations arising from using classical instead of quantum mechanics are considered.