[Pw_forum] Unit cell and atomic position optimization

[Lazaro Calderin]

Dear Yanming,

Thank you very much for your e-mail.

In your previous mail to the forum you wrote:

>Alternatively, I try to fix target pressure, and optimize a,b,c and atomic
>positions using VCSMD. The key input parameters to realize this function for
>Boron are as follows (other common parameters are not shown).
>
>ibrav=9, celldm(1)=8.5,celldm(2)=1.69,celldm(3)=1.00168
>nat=4,ntyp=1,
>Calc='cm',iswitch=3,
>amass=10.811,cmass=43.2,
>nosym=.true.,order=3,press=0.0,
>
>But the final results are not satisfied.

... and this induced me to think that what was not satisfied was the
target pressure. I am sorry I misinterpreted you. Any ways that is the
kind of problem I am getting and I appreciate your answer.

Now, about cm, cmass and "damping". The file INPUT_PW says the following
for calc='cm':

'cm' : Parrinello-Rahman cell minimization by damped dynamics

My understanding is that in Parrinello-Rahman kind of MD the "cell mass"
is an inertia parameter that controls the motion of the unit cell. Therefore, one 
could use it to accelerated or slow down (probably damp is not the right word) the 
convergence of the optimization. If I remember well Andersen gives an optimum of 
3/(4XPi^2)(mass of unit cell) for cmass. I tried this value for my system and larger 
and smaller values but I could not get pw to converge to the target pressure. I also
looked for a real damping parameter but could not find it. It is likely hardwired or 
defined by pw (?). I will keep looking for more info. Please,if you know anything else 
regarding this point let me know.

Thanks again and regards,
Lazaro Calderin