I was reading Computational Materials Science by June Gunn Lee, and in his examples he turned off the symmetry for systems which I feel is symmetrical. I'll give examples:
- In page 202, the system is Pt atom. It was switched off "for better description of an atomic Pt." But why is this the case? Isn't an atom symmetrical?
- In page 221, the system is a Pt(111)-slab and ISYM=0. But why? Isn't there some symmetry in this system since it is a crystal?
Thank you very much.
I don't know the particular examples that you are referring to, but having symmetry switched off will sometimes prevent you from finding spontaneous breaking of symmetries that should occur. Otherwise you are right, I would expect a Pt slab retain its symmetries. But then maybe it doesn't, and for that reason many people prefer leaving it off.
In general, the charge density from an atom does not necessarily have the spherical symmetry of the potential, since you will populate single particle wavefunctions corresponding to the irreducible representations of the rotational group, i.e. the spherical harmonics, which are not in themselves spherically symmetric. What DFT gives for Pt I don't know though, that is a different story.
In principle, if the energy goes down by not using symmetries, then that is the correct thing to do, but since the calculation when not symmetrizing is not numerically exactly the same, the issue can be a little tricky. Do your own experimenting for some system with well-known symmetries to estimate the effect of turning ISYM on and off, say bulk Pt to estimate the numerical effect of changing it. I would expect it to be small though.
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