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Dear all,I want to do some accurate calculations in grain boundary,whose supercells are cleaved from the relaxed bulk.
In the bulk calculation,I could get the external pressure closed to zero.However,in the interface calculation,the external pressure is about -7kb.I don't know whether it has an effect on the total energy of the supercell.

Many thanks!

as long as some stress and external pressure persists on the cell, the total energy will not be at its minimum, because the geometry for that specific system does not correspond to its equilibrium structure.
However, most probably any kind of periodic supercell is just a model for macroscopic grain boundaries, therefore I suppose that relaxing the atomic positions (for a given supercell) and minimizing the total energy with respect to that is the best which can be done on any ab-initio level.