[CPMD-list] SUBTRACT COMVEL ROLVEL and Nose?

[Axel Kohlmeyer]

On Thu, 14 Jul 2005, Lan-Feng Yuan (lyuan at Princeton.EDU) wrote:

LY>   Hi, dear CPMD users,
LY> 

dear lan-feng,

LY>   I want to make sure that the energies from CP and path integral MD
LY> only include vibrational contributions, not translational and
LY> rotational. It seems that the keyword SUBTRACT COMVEL ROLVEL would
LY> subtract global momentuam and angular momentum every given steps. The
LY> manual says that the use of these keywords is strongly recommended for
LY> long runs (e.g. >10 ps) and/or low density systems (e.g. isolated
LY> molecules, gas phase). Since I want to model water monomer and dimer,
LY> its use is "stronly recommended". However, the manual also says that
LY> since the subtracted kinetic energy is put back into the system by
LY> simple rescaling of the ionic velocities, these options is not fully
LY> compatible with NOSE thermostats ... But I surely need Nose chains in
LY> PIMD. So what to do?

actually, SUBTRACT COMVEL/ROTVEL is not supported 
by the PI-MD drivers at all. so there is not much
of a choice. in general, you should be aware of the
fact, that the path-integral code has not been 
significantly updated for years and is somewhat 
'lagging behind' in terms of features compared to
the other MD drivers.

as for using SUBTRACT COMVEL/ROTVEL in combination
with nose-hoover chains. you _can_ use it, but
due to the rescaling, the ensemble is not exactly
preserved anymore. there may be a smarter way of
doing it, but so far nobody has come up with it.
on the other hand, if you use a fequency between 
20 and 100 the rescaling should be rather small. 
in the end it, is a trade-off between keeping the 
correct ensemble and losing kinetic energy into  
rotation and translation.

best regards,
	axel.

LY> 
LY>   Thank you very much! Regards,
LY> 
LY>   Lan-Feng
LY> 
LY> ----------------------------
LY> Dr. Lan-Feng Yuan,
LY> Postdoctoral Associate,
LY> Department of Chemistry,
LY> Princeton University,
LY> Princeton, NJ 08544, USA
LY> Office Phone: 1-609-258-1834
LY> Fax: 1-609-258-6746
LY> ----------------------------
LY> 
LY> 
LY> _______________________________________________
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LY> CPMD-list at cpmd.org
LY> http://cpmd.org/mailman/listinfo/cpmd-list
LY> 
LY> 

-- 

=======================================================================
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/
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.