[Pw_forum] on variable cell optimization

[Serge Nakhmanson]

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Paolo Giannozzi wrote:
> On Friday 28 January 2005 16:58, Serge Nakhmanson wrote:
> 
> 
>>     Entering Dynamics;  it =    28   time =  0.00131 pico-seconds
>>[...]
>>Nothing is moving, nothing is changing.
> 
> 
> in 1.31 fs very little is happening, for sure
> 
> 

Dear Paolo,

Thank you very much for your advice. I understand now what I did
wrong. I did not realize that a timestep is actually attached to
a vc-relax calculation (when I do fixed volume relax there is no
timestep attached to it). I thought dt has to be set only if I
do md or vc-md. A timestep of ~ 0.05 fs is surely too small.

>>Or is my cell too heavy (see below)?
> 
> 
> you used the default value for the time step (1 a.u.=0.0484fs): it is 
> way too small. There shouldn't be a default value for the time step
> in the first place. There is one for the cell mass, and it should be good 
> for most purposes.

Then I was a bit confused by the documentation. This is what is
written it INPUT_PW:

wmass          REAL
               ficticious cell mass for variable-cell md simulations

- From this I assumed that there is no default for wmass and I do
have to set it something.

> 
>>BTW, what is the UMA unit ( cell mass =  36.00000 UMA )?
> 
> 
> atomic mass units, I guess: mass of H=1
> 

Shouldn't we call it AMU or AUM then? :)

S.



- --
******************************************************************************
 Serge M. Nakhmanson                                 phone: +1 (732) 445-4603
 Postdoctoral RA                                       fax: +1 (732) 445-4400
 Department of Physics & Astronomy
 Rutgers, The State University of New Jersey
 136 Frelinghuysen Road                          nakhmans at physics.rutgers.edu
 Piscataway, NJ 08854-8019          http://www.physics.rutgers.edu/~nakhmans/

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