[CPMD-list] Energy components in PIMD

[Axel Kohlmeyer]

On Mon, 5 May 2008, ukasz Walewski wrote:

dear lukasz,

LW> Dear Axel,
LW> 
LW> I just wanted to give you an update on my progress. I did the NVE runs
LW> with USPP, CUTOFF=30 Ry and DUAL=8,10 and also with MT, CUTOFF=70 Ry and
LW> DUAL=4. The EKINC has very similar drift as for previous runs, i.e.
LW> USPP, CUTOFF=30 Ry and DUAL=4 - see
LW> http://bioexploratorium.pl/ljw/cpmd/nvt/test1/EKINC.png and
LW> http://bioexploratorium.pl/ljw/cpmd/nvt/test1/EKINC.agr

without seeing the corresponding development of the
ion kinetic energy and the potential energy there is
very little to say. from running your input briefly it
looks as if your choice of reference frequency for the
ionic nose-hoover chains does not couple well to the
prevalent modes in your system (how did you determine it?).

i did some runs of the input by myself (on a very old,
soon to be decomissioned cluster, so it took a loooooooong 
time). i reduced the time step and played with the GC-CUTOFF
parameter and the thermostat coupling and my conclusion is 
simply that with the ultra-soft pseudopotential support as 
it is currently implemented in CPMD, you cannot fully conserve
the fictitious kinetic energy. with your input i always get
a drift of roughly 0.0005 Ha/ps in EKINC. with norm-conserving
pseudopotentials, however, this does not happen and once
i actually run across a large enough number of nodes the 
NC-run is almost as fast as the USPP run...


LW> The same directory contains the input files:
LW> http://bioexploratorium.pl/ljw/cpmd/nvt/test1/uspp-dual4.inp
LW> http://bioexploratorium.pl/ljw/cpmd/nvt/test1/uspp-dual8.inp
LW> http://bioexploratorium.pl/ljw/cpmd/nvt/test1/uspp-dual10.inp
LW> http://bioexploratorium.pl/ljw/cpmd/nvt/test1/mt-dual4.inp
LW> 
LW> 
LW> [...]
LW> 

LW> I have run out of the ideas on what can be wrong...

one thing you also have to look out for is the time step. 4.0 
could be a bit on the large side whenever you have a proton transfer
as this would suddenly induce a larger change in the electron structure.

cheers,
   axel.

LW> 
LW> Kind regards,
LW> Lukasz
LW> 

-- 
=======================================================================
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
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
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If you make something idiot-proof, the universe creates a better idiot.