[Pw_forum] Dispersion correction issue

[Michael Fischer]

Dear Martin!

You are indeed right, that's the origin of my "problem". Stupid error on 
my part. Thanks for the clarification!

Best wishes,
Michael



Martin Andersson wrote:

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
 >
 >