[Pw_forum] Estimate time for relaxing bigger unit cells?

[dario rocca]

Dear Tomas,
Since you need to relax several 64 atoms cells the best estimate is to
relax it once to be sure that it converges and to have an accurate estimate
of the time. However, consider that a 64 atoms cell close to equilibrium
might converge much more rapidly than a cell far from equilibrium.
A calculation involves several different mathematical operations with
different scaling. When the system is big the operations with the worst
scaling dominate. When the system is small instead even an operation with a
good scaling can take an important percentage of the total computer time.
I'm not sure I would try to guess the time for 64 atoms from a 8 atoms
cell.
Best,
Dario

On Fri, Oct 14, 2016 at 5:04 PM, Rojas Solorzano, Tomas <tr074112 at ohio.edu>
wrote:

> Dear QE community,
>
>
> I am looking for a way to get a rough estimate of the time for a
> relaxation of a 64 atoms unit cell based on the time that takes a 8 atom
> unit cell of the same system. I am supposed to request for an allocation in
> the cluster and for my project will need to relax several unit cells of
> that size.
>
> I am aware of the formula in the documentation to estimate the
> SCF calculations but I though there might be tips for the relaxation. My
> initial try was to relax the 64 unit cell for a few hours and estimate it
> by comparing to the small one, but I am not sure how.
>
>
> Regards and Thank you!.
>
>
> Tomas R
>
> Ohio University
>
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