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Hi everyone!

In order to make some tests I tried to do modelling of a single H2 molecule in a square box of 10Ang.

Depending of its position in the box, the energy of H2 changes after optimization. The energy difference is about 0,0005eV.

Since this is a periodic system, why isn't the energy the same wherever H2 is in the box?

1) the results should be independent of the position of H2 in the box. Please check whether
--) the difference is significant at all (ionic and electronic convergence criteria)
--) you chose the correct KPOINTS (Gamma only)
--) the symmety analysis obtained equal results

Thanks for the answer.

Just to make sure I understand your second point:

By choosing the correct KPOINT "gamma only", do you mean to specify Gamma on the third line of the KPOINT file?

simply give a 1x1x1 MP mesh, this generates exactly the gamma point