[Pw_forum] how to convert to unit of eV/atom in supercell?

[Gabriele Sclauzero]

Dear Kelvin,

   please make a serious effort to include in your responses only relevant emails: 
replying to digests is quite not a good idea... I had to scroll hundreds of lines before 
getting to the right point.


>     Message: 4
>     Date: Thu, 04 Feb 2010 09:08:41 +0100
>     From: Gabriele Sclauzero <sclauzer at sissa.it <mailto:sclauzer at sissa.it>>
>     Subject: Re: [Pw_forum] how to conver to unit of eV/atom in supercell?
>     To: PWSCF Forum <pw_forum at pwscf.org <mailto:pw_forum at pwscf.org>>
>     Message-ID: <4B6A8089.9080009 at sissa.it
>     <mailto:4B6A8089.9080009 at sissa.it>>
>     Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> 
> 
>     Wong Kelvin wrote:
>      > Dear all
>      >
>      > when i calculate a single atomc Cu primitive cell, the energy is
>     ~ -87
> 
>     What are you trying to compute? An isolated atom or a bulk phase? In
>     any case, if you are
>     using pseudopotentials the total energy has not any physical meaning
>     by itself. What is
>     physically relevant are energy differences.
> 
>      > Ry. How to convert it to the eV/atom which normally is around
>     -3.7 eV?
> 
>     Where did you get this value of -3.7 and what does it actually refer
>     to? I mean, to which
>     physical quantity?
> 
> 
>     GS
>      > Thanks
>      > Wong
>      >
>      >
>     o ------------------------------------------------ o
>     | Gabriele Sclauzero, PhD Student                  |
>     | c/o:   SISSA & CNR-INFM Democritos,              |
>     |        via Beirut 2-4, 34014 Trieste (Italy)     |
>     | email: sclauzer at sissa.it <mailto:sclauzer at sissa.it>              
>               |
>     | phone: +39 040 3787 511                          |
>     | skype: gurlonotturno                             |
>     o ------------------------------------------------ o
> 
> Dear Gabriele
> Thanks for reply!
>  
> I am trying to calculate the grain and twin boundary energies in metals. 

I don't think that any code can give this energies with a single total energy calculation. 
You have first to learn the general methodology to build these quantities.

> I partially understand that absolute energy in pseudopotential 
> calculation does not provide meaningful interpretation. 

If you are dealing with a bulk phase it's not only a matter of pseudopotential, since you 
are considering an infinitely extended solid.
On the other hand, all electron energies of an isolated atom, if studied in a large enough 
supercell, I think they would have a physical meaning.

> The reported energies in literature for copper with PP calculations use 
> to be around -3.7 eV/atom. Is there anything significant abt this 
> figure? and i do not understand how to get it. simple by diving the 
> total energy calculated by number of atom in supercell? if it is so, the 

Although I never did such kind of calculations, I don't believe that it's so simple to get 
that number. You should learn this from some textbook or lecture notes, rather than from 
this forum (which should treat more technical issues). Maybe if you are lucky some user 
with more experience on this will give you advice.


GS

> value (~3.7) is way too small as compare to the 87 Ry for one copper 
> atom primitive cell.
>  
> regards
> Kelvin 
>  

-- 


o ------------------------------------------------ o
| Gabriele Sclauzero, PhD Student                  |
| c/o:   SISSA & CNR-INFM Democritos,              |
|        via Beirut 2-4, 34014 Trieste (Italy)     |
| email: sclauzer at sissa.it                         |
| phone: +39 040 3787 511                          |
| skype: gurlonotturno                             |
o ------------------------------------------------ o