[Pw_forum] About energy of a single atom

Eyvaz Isaev eyvaz_isaev at yahoo.com
Sun Feb 27 23:40:32 CET 2005


--- Shujun Hu <hushujun at mail.sdu.edu.cn> wrote:
> I tried to calculate the energy of a single atom in
> order to gain the bond energy.

It is not so clear what do you mean. Did you calculate
the energy using atomic program or using  pw.x and
supercell approximation?

> Although such information is given in the PP, i
> donnt trust it. 

Sorry, but I don't see any information about bond
energy in a PsP file, say, for Al. There is just the
total energy for atomic configuration used for PsP
generation. So, it will be different for different
atomic configurations. And methods, too.

Do not trust, do not use.

>One of the  important parameters is the atomic 
> distance. Maybe 10 times of the atomic radius
> between atoms is better, and such system may be
> considered as "alone". Yes or no?
> And the E-cutoff, how to set?

The distance could be approved just on the base of
tests with different size of a supercell where there
is your atom. 
The choice of the cutoff energy (ecutwfc) depends on a
pseudopotential in use. If you would like to use
norm-conserving pseudopotential for 3d-metals, and the
first-row elements (like C, O), you should use cutoff
energy around 100Ry to get good results. For test
purpose ~80Ry is OK, I believe. For ultrasoft
pseudopotentials usually the energy is around 25-35Ry.



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