[Pw_forum] Absolute energy dependent on ATOMIC_POSITION specification

[Paolo Giannozzi]

On Thu, Apr 13, 2017 at 10:20 AM, Daniel Reta
<daniel.reta at manchester.ac.uk> wrote:

> I have run the same calculation specifying the atomic position with "crystal" and with "angstrom", and I got two different absolute energies. Is this correct?

make a "diff" of the two outputs and you will find that they are not
"the same calculation"

> Also, the system is a molecular crystal with some ligands coordinated to Ytrium. I find that I have to go to very high values of ecutwfc to converge the absolute energy. I'm talking about 100 or 110 Ry approximately. Is this a known feature?

the convergence of absolute energies may be very slow, depending upon
the pseudopotentials used. Energy differences between different
structures of the same material typically converge much better. This
is a known feature, yes.

> Does working with such high values of ecutwfc introduce some biases in the calculations?

no, it just requires a big computer and a lot of time

Paolo
> Best regards
>
>
> ========================
>
> Dr. Daniel Reta,
> Post-doctoral Research Associate,
> Computational Chemistry, School of Chemistry
> University of Manchester
>
> Email: daniel.reta at manchester.ac.uk
>
> ========================
>
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222