Perhaps this paper could help. 

The answer depends on the level of detail you require for your simulation. One can work at the quantum level or at the lattice level, or at the micromagnetic level, or at the bulk level. Depending on your application, you choose your model and the corresponding numerical method - there is no universal answer to your question, unfortunately.

There are many, Comsol, matlab and .... (finite element method)

Perhaps this presentation could help you. 

We develop a model of magnetic permeability and then fit the parameters of model with genetic algoritms method.