[Pw_forum] Unit cell and atomic position optimization

[John Teller]

Dear Lazaro,

>I got pwscf running on a SUN cluster a while ago and I have started
>doing some optimization of both unit cell parameters and atomic
>positions in a hexagonal system (hydroxyapatite) to compare with LCAO
>results. I have a problem similar to the one posted by Yanming Ma in >his 
>last message. That is: pwscf does not converge to the a given >pressure.
>I tried increasing the mass of the unit cell to damp the oscillations
>between negative and positive values of the pressure but it did not work
>neither. I would be interested in any suggestions on how to perform
>relaxation of unit cell parameters and atomic positions at the same time
>in non-cubic systems using pwscf.

Actually, I did not state that pwscf does not converge to a given pressure. 
I just mentioned that by comparing with other ab initio plane wave code 
(VASP or ABINIT), PWSCF may be less convient to get the EOS for non-cubic 
system, based on my limited knowledges on PWSCF. So I sent my questions on 
how to get the proper input parameters when I try to do geometry minisation. 
I also want to make sure what kind of MD ('cm','nm','mm')is the best to do 
geometry minisation. But nobody reply me except Guido
>From my opinion and experience, the key to do geometry minisation is how to 
choose the input parameter of 'cmass', no matter what kind of VCSMD you use 
e.g., 'nm','cm' or 'mm'. The values of cmass for these kinds of MD are quite 
different. I can get a good result for geometry minisation using 'cm' method 
(I have shown the input parameters in my last email).  The cmass in 'cm' 
should be the real cell mass from my experience. While cmass in 'mm' could 
be very small(say 0.0001), as I discussed with Guido , but i don't have any 
experience for 'nm' and 'mm' MD. I guess that the reason you can not get a 
good minisation at a give pressure is because you choose a wrong CMASS, also 
it doesn't make sense that "increasing the mass of the unit cell to damp the 
oscillations..". It depends on what kind of MD you use.
So you can try to do geometry minisation using 'cm' and, if possible, let me 
know your results.But sometimes I got error message "cdiaghg: error", I 
haven't figure out yet what is the reason for the error.

Best Wishes

Yanming Ma
Steacie Inst. for Molecular Sciences
Naional Research Council of Canada


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