[Pw_forum] Calculation of Gibbs free energies
GAO Zhe
flux_ray12 at 163.com
Wed Jul 11 11:54:42 CEST 2012
According to my experience, the general process is right, but, this general process is normally working for crystal rather than amorphous, liquid and gas. Since the contribution from vibration is not that much in those matter, for example, transition contributes much more in gas phase. Therefore, Monte Carlo or Molecular Dynamic could be better choice in these cases.
Technically, in your calculation, H2O molecule may exist in a box, therefore only the Gamma phonon calculation is meaningful. Since q2r means fitting the finite samplings of dynamic matrics to infinite interatomic force constants, only one point is not that reliable to find good aproach of IFCs. Actually, since vibrational free energy is defined with the term of hv, you can derectly obtain Fvib from your first step.
--
GAO Zhe
CMC Lab, Materials Science & Engineering Department,
Seoul National University, South Korea
At 2012-07-10 22:44:56,"Torstein Fjermestad" <torstein.fjermestad at kjemi.uio.no> wrote:
> Dear all,
>
> In one of my projects I would need to compute the Gibbs free energies
> of structures along a reaction path. After looking through the QE
> webpage, the pw_forum, and the documentation in the quantum espresso
> installation directory I think I understand the general procedure:
>
> 1. one calculates the phonons
> 2. one computes the interatomic Force Constants with the q2r.x program
> 3. one uses the matdyn.x to produce input for the F_QHA.x program
> 4. with the F_QHA.x program one can compute the Gibbs free energy and
> other thermodynamical properties.
>
> (please correct me if I have misunderstood this general procedure)
>
> The problem is that I have so far only managed to complete step 1, to
> compute the phonons. I am currently doing test calculations on a water
> molecule and this is the input file of the phonon calculation:
>
> phonons of H2O in super cell at Gamma
> &inputph
> outdir='./',
> prefix='h2o',
> amass(1)=1.01,
> amass(2)=16.01,
> fildyn='h2o.dynG',
> trans= .true.
> /
> 0.0 0.0 0.0
>
> I then try to run the q2r.x calculation with the following input:
>
> test of q2r calculation
> &input
> fildyn='h2o.dynG',
> flfrc='h2o-output-q2r.fc',
> zasr='no'
>
> This calculation crashes presumably because it expects a file called
> h2o.dynG0. But from where do I get this file? As far as I can see the
> phonon calculation only produces the file h2o.dynG, not a h2o.dynG0
> file.
>
> I am completely lost at the moment so I would appreciate if anyone
> could help me with carrying out the q2r calculation correctly.
>
> Thank you very much in advance,
>
> Yours sincerely,
>
> Torstein Fjermestad,
> University of Oslo,
> Norway
>
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