\ 

An elementary cell is a cube of edge length l0
and pairs of nearest neighbour cells are called `doublet'. For each
individual physical process that we take into account, there is a set
of doublet transitions.
Each transition is associated with a rate parameter with units of frequency that introduces into the model the characteristic time scale of the corresponding physical mechanism. Doublets are all supposed to be independent and the occurrence of a transition of doublet is assumed to follow a stochastic process. 
