[Pw_forum] vc-relax newbie question

obm mbaris at metu.edu.tr
Mon May 28 16:16:57 CEST 2007


First of all, thank you very much for the quick reply,

at this point I will no longer try using vc-relax for this problem, as
Paolo suggests (let the scripting begin).

However, I wish to comment the suggestions Cesar R.S. da Silva made, at
least for reference purposes

1-use default verbosity
I tried all verbosity levels, all to no avail (looking down, possibly
the problem was different)
2-wmass is way too high. Use something like 0.002
I am completely puzzled at this. Looking at the help document INPUT_PW
wmass          REAL ( default = sum of atomic masses in the cell )
               fictitious cell mass [amu] for variable-cell simulations
               (both 'vc-md' and 'vc-relax')
being a fcc crystal, there are 4 atoms in the cell with mass 106.42 amu
each summing to indicated value. I can only imagine your suggestion is
based on a deeper understanding of the relaxation procedure which I
failed to find a reference to in the manuals. I would be grateful if you
suggest one.
3-In Electrons namelist use conv_thr at least as small as conv_thr=1.0d-8.
You are absolutely right.
4-in Cell namelist, you have to set the target pressure (like   press=0.0).
Again in the manual INPUT_PW it states 0.0 is the default value.

press          REAL ( default = 0.D0 )
               target pressure [KBar] in a variable-cell md or
relaxation run.

Do I have to include cards, even I will use the default values?

5-dt has to be set in Control namelist. dt=80 will probably do OK.
So, does vc-relax rely on some molecular dynamics? I thought it was
similar to implementation in gaussian03, where the program does the
volume to energy fitting for you (in a rather efficient manner) and
returns the final cell parameters. Well, if this is the case, it really
does not fit my purposes.

Thank you again,

O Baris Malcioglu.

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