As lammps manual introduced,the "fix orient/fcc" command is used to simulate the mobility of fcc material under the additional potential energy, so i want to know whether the configuration ,i.e.the position of each atom,used to simulate GB mobility is the configuration corresponding with lowest GB energy and via the "read_data" command to read in? But i still do not know how to use "fix orient/fcc" command to simulate mobility,i.e. the difficulty of editing source code.
Not sure what you are asking, or what you mean
by "editing source code". You do need to input
a geometry for an fcc material with a grain boundary
in it, e.g. a bicrystal. Then you just use the fix
and it applies the driving force and you observe
the boundary move, extracting the mobility.