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I have a question about the structure relaxation:
usually using the IBRON=1/2, the algrithm will try to optimize the atom position and get to the global/system minimum energy. However i am thinking whether there is a way to find a local minimum energy. WHich means that the structure relaxation may stop at some energy valley, but not the minimum system energy.
thanks

The optimization will usually stop at a local minimum every time. It will only give you a global minimum if you started the calculation with a good guess. For instance, if you do a calculation for GaAs with your atoms positioned initially close to what would be a wurtzite lattice, then the calculation will converge to the wurtzite arrangement, even though the most stable structure for GaAs is zinc-blende.

In short, your ability to find a local minimum depends on how close your starting configuration is to that minimum.