Gamma point only versus other KPOINT grid calculations

[SKM]

hi
I wish to ask a few points regarding geometry optimisation regarding Gamma only [1 1 1] and, say [3 3 2] or any such multi grid KPOINTS;

1. The optimised total energy in both cases will be different, and that's the reason we do convergence test to know which grid gives us minimum energy. Right?
2. In such case, how would the Gamma point only calculation will help us? Which occasions we can use those results?
3. Can I take the electronic reorganisation during Gamma point only optimisation as similar to that of at different KPOINTS grid?
4. Can we pickup the Gamma point only energy profile from the output of other KPOINTS grids, say [3 3 3]?
If we can, then how to pick the values? And will this energy be same as that got from Gamma point only calculation?

Regards

[henrique_miranda]

1. Yes
2. You can use a Gamma point only when the final results don't change (within your own criteria) when you use denser k-point grids.
3. I don't know what you mean by electronic reorganization. If you mean charge density (CHGCAR) then yes.
4. No. The charge density and total energy is computed by summing over the brilluin zone. In principle one could compute the Gamma only energy from a 3x3x3 k-point calculation but this feature is not implemented and most likely it will never be.

Kind regards,
Henrique Miranda

[SKM]

hi
Thank you for the reply. based on your reply follow-up queries are below, please:

2. You can use a Gamma point only when the final results don't change (within your own criteria) when you use denser k-point grids.

do you mean during convergence test, if the total energy doesn't change MUCH comparing Gamma and dense grid, then I Can use Gamma only calculations?

2A. Even otherwise, 'what is', and 'where do' we use Gamma point only calculation?

3. I don't know what you mean by electronic reorganization. ....

Sorry. what I meant was the ionic (atomic) restructuring (or reorientation) resulted after optimisation. (i.e. optimised) can be similar to that of dense grid?

4. No. The charge density and total energy is computed by summing over the brilluin zone. In principle one could compute the Gamma only energy from a 3x3x3 k-point calculation.....

as my query is regarding total energy of Gamma only from dense grid, is it not the same (in principle) if i take the energies at different bands from band structure and sum-up at Gamma point only from the dense grid?

regards

[ferenc_karsai]

2) The larger your supercell the more likely it is that a Gamma point only calculation is sufficiently accurate.

3) See point 2.

4) No, total energies are not just a sum of eigenvalues.