[Pw_forum] Dispersion correction issue

[Martin Andersson]

Hi,

QE uses Ry. I believe that CASTEP uses Ha. There is your factor 2.

Cheers,
Martin Andersson
University of Copenhagen


On 8 nov 2013, at 12:37, Michael Fischer <dommiguel at gmx.de> wrote:

> Hello everyone,
> 
> I am trying to do some single point calculations for a porous system 
> including a guest molecule using QuantumEspresso 5.0.2, employing the 
> PBE functional and the Grimme-type dispersion correction (London=.true., 
> other related parameters on default settings).
> 
> When comparing my results to earlier calculations using CASTEP with 
> comparable settings, I found rather large differences, and it became 
> clear quite quickly that the majority of these differences are due to 
> the dispersion interactions. Below, I have extracted the dispersion 
> contribution to the total energy (in Ry) obtained with the two different 
> codes (sorry for awkward formatting):
> 
>                                CASTEP QuantumEspresso
> Zeo                          -0.11569 -0.23136
> Guest                      -0.00010                        -0.00020
> Zeo + Guest            -0.12646                        -0.25290
> 
> It turns out that the dispersion energy from QuantumEspresso is 
> practically exactly twice as large as the energy calculated by CASTEP. 
> Looking at previous literature studies, I have good reason to "trust" 
> the CASTEP energies (they are at least in the right ball park), while I 
> must assume that the QE results are too large. Is this problem known to 
> anyone?
> 
> I should possibly add that I successfully ran the test that involves the 
> Grimme-type correction (the dispersion contributions in vdw.out and 
> vdw.ref are the same).
> 
> Any help will be greatly appreciated.
> Kind regards,
> Michael
> 
> 
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