[Pw_forum] Pressure calculations at gamma (version espresso 2.1.4)

[merlin.meheut at lmcp.jussieu.fr]

Dear pwscf authors,

I wanted to signal a strange feature of pressure calculations when you put the
option K_points {gamma} with scf calculations. My system is a water molecule
inside a supercell of 30 Bohr radius. I recognize that pressure has no real
physical sense in this system.

Nevertheless, I am surprized that the pressure calculated with k = 0, with the
espresso 2.1.4 version (-20 kbars), is such different from the pressure
calculated:
- at k=0, with the version pw.1.3.0
- at k different from zero (I tried k=0.5 0.5 0.5 , and k=0.0, 0.0, 0.5), with
the  espresso 2.1.4.

In the two last situations, the found pressure is of -0.1 kbars (let's say 0.0).

I can also mention that this pressure of -20 kbars is quite stable with
convergency criteria and cutoff frequencies (from ecutwfc=60 to 130Ry and from
conv_thr= 10-7 to 10-9).

I think that we can conclude to an effect due to gamma-point specific
algorithms.

I don't know if it can create troubles in other circumstances, actually for
other reasons I will not continue to investigate this trouble ( I am
calculating phonon frequencies, and ph.x refuses to work with pw.x data files
calculated at gamma:
     from phq_readin : error #         1
     cannot start from pw.x data file using Gamma-point tricks )

My input files are partially contained in a precedent post.

Greetings,

Merlin


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