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optimize geometry

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tara95:
hi
please  explain to me that,
Is it necessary  to optimize the k-points and cut-off energy parameters before optimize geometry?

Anders Blom:
Generally yes, and most often the optimization will converge faster if you have sufficient accuracy, because the forces will be more accurate, and the forces are actually more sensitive to settings like k-points than the total energy is. However, sometimes it can save some time to use slightly lower accuracy settings for the first part of the optimization, maybe converge to higher force tolerance like 0.05 eV/A, and then use finer settings to converge the last bit to 0.01 eV/A or so. But the safest option is to ensure good accuracy first, and then run the optimization with that.

tara95:
thank you :)
Another question;
Which basis set(HGH-FHI-OMX) is more suitable if I use GGA for optimize geometry? And should optimized parameters be used to band structures?

Jess Wellendorff:
The choice of pseudopotentials+basis set depends in general on the system under investigation. So it's often a matter of testing. Generally speaking, OMX will be better than FHI, but also computationally more demanding. And yes, once you chose a set of parameters, stick to those for subsequent band structure calculation also.

Anders Blom:
OMX is harder, you need a considerably larger mesh cut-off for those, but I agree they are often (not always) of better quality than FHI.
I would however recommend SG15 as a general-purpose choice.

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