[CPMD-list] How to rerun the same job for statistical purpose?

[Axel Kohlmeyer]

On Wed, 8 Dec 2004, Archilles wrote:

dear peng,

AP> Dear all,
AP>  
AP> 
AP> I tried to rerun the same CPMD simulation job several times to obtain
AP> statistical sense of multiple simulations. By "the same CPMD
AP> simulation job ", I mean the identical input file. But what I got is
AP> the identical output files.

well, if you start every time from the same coordinates and 
electron structure (i.e. the same RESTART file), you should get
the same trajectory for quite a while until some numerical
noise kicks in and the trajectories will gradually diverge.

if you use the restart written at the end of each simulation
to start the next run, there should be differences.

AP> After changing the temperature by 1 K, each step KS energy remains the
AP> same with two different runs.

that depends on what kind of thermostatting you use.
with TEMPCONTROL IONS for example, the change by 1K will
only affect the simulation, if the instantaneous temperature
with be outside of the given bounds.

it may be helpful, if you'd post your input (or at least the
part up to the coordinates section, if you have a large system).

AP> The trajectory also remains the same for two runs. 
AP> 
AP> What I expected is that each run are simillar, but quite different
AP> from each other. Maybe each run the program use the same random seed
AP> to start the simulation, which leads to the same results. Is there any

why a random seed? you are doing molecular dynamics, in which the
new positions are determined from the old positions of the atoms
and the forces on them.

AP> way to change it each time? Therefore, I can run the same job more
AP> than once, and get different results to inspect the statistical
AP> properties of the system.

due to ergodicity hypothesis, you only have to run the MD long 
enough to sample phase space, povided you are in equilibrium,
and the average over them. of course you could also use different
starting coordinates and thus have independent trajectories, but 
those you'd have to create yourself.

regards,
	axel kohlmeyer.

AP> 
AP> Thank you very much.
AP> 
AP> Peng
AP> _______________________________________________
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AP> 
AP> 

-- 

=======================================================================
Dr. Axel Kohlmeyer   e-mail: axel.kohlmeyer at theochem.ruhr-uni-bochum.de
Lehrstuhl fuer Theoretische Chemie          Phone: ++49 (0)234/32-26673
Ruhr-Universitaet Bochum - NC 03/53         Fax:   ++49 (0)234/32-14045
D-44780 Bochum  http://www.theochem.ruhr-uni-bochum.de/~axel.kohlmeyer/
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If you make something idiot-proof, the universe creates a better idiot.