[Pw_forum] Calculation of Gibbs free energies

[Lorenzo Paulatto]

On Tue, Jul 10, 2012 at 4:44 PM, Torstein Fjermestad <
torstein.fjermestad at kjemi.uio.no> wrote:

>  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:
>
> ...

 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.
>
>
 Dear Torstein,
the problem in your case is that q2r.x expects to find a phonon
*dispersion* calculation, i.e. the phonon computed over a regular grid of
q-points n reciprocal space. On the other hand, you only computed the
phonons at Gamma. It is true that Gamma is just a 1x1x1 grid, and if you
are sure it is enough to describe your system you can manage to convince
q2r to use it. You need to do the following:
1. move h2o.dynG to h2o.dynG1
2. create a file called h2o.dynG0 which this inside:
 1 1 1
 1
 0.0 0.0 0.0
3. try again with q2r.x

Alternatively, you can just repeat the phonon calculation with ldisp =
.true.; if you want your grid to be just the Gamma point you specify nq1=1,
nq2=1, nq3=1.

I stress that a single-point sampling may not be enough to describe
accurately the properties of your system (unless the supercell is very
large) but this you need to check for yourself.

good work


-- 
Lorenzo Paulatto IdR @ IMPMC/CNRS & Université Paris 6
phone: +33 (0)1 44275 084 / skype: paulatz
www:   http://www-int.impmc.upmc.fr/~paulatto/
mail:  23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05
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