[CPMD-list] how to get a equilibrium state at finite temperature?

[Axel Kohlmeyer]

On Mon, 21 Nov 2005, Chen Ying wrote:

CY> Dear all

dear chen ying,

CY>  I used the following steps in order to get a equilibrium state at 4K.
CY> 1. wavefunction optimization
CY> 2. a short MD runs at 10K, because the phase transition happens at 13K.

please not that phase transitions are subject to finite size effects
and that depending on how accurately your system can be described with
planwave/pseudopotential + DFT, the transition temperature can be pretty
far off.

CY>    TEMPERATURE
CY>    10.0
CY>    TEMPCONTROL IONS
CY>    10.0 2.0

using a delta of 2K here is pretty pointless, as your 
temperature is likely to flucuate in equilibrium much
more. note, that for a system of 32 water molekules (~100 atoms)
the fluctuation in the instantaneous temperature is typically
around +/-50K at 300K. see my previous post on cpmd-list for
a handwaving explanation.

CY> 3. a relatively long MD runs at 4K

what is 'long'? how many ps?

[...]

CY> the problem is that EKS keeps reducing (but very slowly) during the
CY> MD runs. I wonder whether i have used the wrong method to relax the
CY> configuration, or I should take more runs until EKS becomes stable.

no. you are basically caught between a rock and a hard place.
equilibration at low temperature takes extremely long. there are
little fluctuations to get the system across barriers to find the
lower minima. the fact that you essentially kill all fluctuations
with the small delta in TEMPCONTROL IONS makes it worse. your system
is not in equilibrium. i suggest you try using a higher temperature
(e.g. 30K or 50K) to see whether the phase transition actually does
happen, run that for a while, perhaps keep a few restarts from that
trajetory and try if one of them  gives you an reasonably 
good starting point to get to an equilibrated configuration.

other than that, you may want to have a look at the tricks
that people do, who simulate glasses whether there is a useful
idea in it for you. most likely, you'd have to add the functionality
to cpmd, though...

regards,
    axel.
CY> 
CY> Thanks in advance
CY> 
CY> 
CY> 
CY> 
CY> _______________________________________________
CY> CPMD-list mailing list
CY> CPMD-list at cpmd.org
CY> http://cpmd.org/mailman/listinfo/cpmd-list
CY> 

-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
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