[CPMD-list] Energy conservation

[Dr. Balasubramanian Sundaram]

Hi Nick,
   In addition, a couple of things came to my mind. Sorry if you have
thought/taken care of these before.

    -- Has the system been "equilibrated" using MD with empirical
potentials,
       prior to the CPMD run? That might help. Since the operating
temperature
       is reasonably large, I am assuming that the system is not
crystalline,
       and that you have an initial configuration that is not obtained
from
       experiments. Hence, my concern about equilibration being a
possible 
       reason for relatively poorer energy conservation. 
       
    -- Is the total potential energy (E_{KS}?) of the system too
exhibiting
       a monotonic decrease? If yes, then the monotonic decrease in the 
       conserved energy may not be from "improper" choice of
timestep/thermostat
       parameters. May be, the system is just relaxing. 

Regards,
Bala


Juerg Hutter wrote:
> 
> Hi Nick
> 
> the problem with energy conservation with systems of this
> type is often related to the thermostats.
> Instead of reducing the timestep you should try to
> increase the order of the integrator and the
> decomposition ratio for the thermostats.
> You do this by setting
> 
> NOSE PARAMETERS
>  4 4 4 6  125 4
> 
> other choices for the Suzuki integrator are
> 3,5,7,9,15,25,125,625
> 
> The decomposition ratio is by default 1 (last number)
> and I did set it to 4. I don't have too much experience
> with that.
> 
> regards
> 
> Juerg
> 
> ----------------------------------------------------------
> Juerg Hutter                   Phone : ++41 1 635 4491
> Physical Chemistry Institute   FAX   : ++41 1 635 6838
> University of Zurich           E-mail: hutter at pci.unizh.ch
> Winterthurerstrasse 190
> CH-8057 Zurich, Switzerland
> ----------------------------------------------------------
> 
> On Wed, 15 Oct 2003, Nick Cooper wrote:
> 
> > Hello all,
> >
> > I am simulating covalent materials at relatively high temperatures (3500K)
> > and I have noticed that the conserved quantity in a canonical ensemble is
> > monotonically decreasing, though admittedly the absolute change is small.
> > I have tried decreasing the timestep (from 3au down to 1au) but all this
> > does is decrease the gradient of the drop in energy. The systems I am
> > looking at have significant metallic character at this temperature, so I
> > have used a Nose-Hoover themostat for the ions and wavefunctions.
> >
> > The monotonic nature is what has me worried- am I just being paranoid?
> >
> > Any advice you can offer would be gratefully received.
> >
> > Yours sincerely,
> >
> > Nicholas Cooper
> >
> > PhD Student,
> > The University of Sydney,
> > NSW 2006 AUSTRALIA
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> >
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