[Pw_forum] How to compare total energy calculation results with references?

[Stefano Baroni]

On Jul 27, 2007, at 12:13 AM, Tengfei Luo wrote:

> Dear all:
> I'm new to PWscf. I tried the first example which includes the  
> total energy calculation of Si. The result I got from this example  
> was:
>
> !    total energy              =   -15.84452726 Ry
>
> I believe this is for the total energy of the two Si atoms system.  
> However, from the reference data here:
>
> http://physics.nist.gov/PhysRefData/DFTdata/Tables/14Si.html
>
> The total energy is -288.198397 a.u.. The caluclated data is very  
> different from this. Is this because of using of pesudopotential?

YES

> How can I get the real totoal energy?

1) Do an all-electron calculation
2) If you want something aproximate, you may want to add to the PWscf  
total energy twice the difference berween the all-electron energy of  
an isolated atom and its energy calculated with the _same_  
pseudopotential that you use for the solid-state calculation. Best if  
the difference is calculated between the all-electron atomic energy  
obtained by a real-space code and the pseudopotential atomic energy  
calculated with plane waves (you can do that with PWscf, with some  
care, if you wish). The reason is PW calculations introduve extra  
numerical errors due to the limited basis set.

>  I would appreciate any help!
>
> Thank you!


you are most welcome

>
> Tengfei

please, do not forget to add your affiliation to your signature

Stefano


---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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