[CPMD-list] Energy components in PIMD

[Axel Kohlmeyer]

On Tue, 29 Apr 2008, ukasz Walewski wrote:

LW> Dear All,

dear lukasz,

LW> I have problems with total energy conservation in PIMD. All the EHAM
LW> energy components are conserved except for ECLASSIC which systematically
LW> drifts. The plots from my simulation are available at
LW> http://bioexploratorium.pl/ljw/pimd/ I use CPMD v. 3.11.1 compiled with
LW> IFC 10.1.008, ACML 4.0.1 and OpenMPI. Attached at the end of this e-mail
LW> is the full input file.

please note that ifort 10.1.008 is _very_ buggy. you should update
to the latest patchlevel, as with all intel compilers...

[...]

LW> Please note that I use termostat both for ions and electrons, so I would
LW> expect kinetic energy to by conserved in such a job.

see... i guess here is the basic mistake. have you done any simulations
_without_ a thermostat? if you have them on both electrons and ions
you are _completely_ blind as to whether you are actually simulating
reality or just generating entropy.

so before doing anything else you should verify the following:

can your input conserve energy for a non-PIMD run?
...and then you have to check whether you can maintain
adiabacity with the massive thermostat on the ions only.
quite a few times, the no-thermostat CP-dynamics is only
meta-stable and when you switch to massive N-H chains you
excite DOFs that will make the CP dynamics lose it.

then you should try PIMD without a thermostat on the 
electrons. that has to be reasonably stable as well.
when switching to PIMD you have to realize that your
"atoms cores" become lighter and move faster, so you
will most likely have to reduce the time step. it
is also quite tricky to make sure that the PI-DOFs
and the fictitious electron DOFs don't couple.

[...]

LW>   ANGSTROM
LW>   SYMMETRY
LW>     8
LW>   CELL ABSOLUTE
LW>     13.0 13.0 8.0 0.0 0.0 0.0
LW>   CUTOFF
LW>     30.0

hmmmm.... 30ry cutoff?


[...]
LW> 
LW> &ATOMS
LW> *N_VDB_BLYP.psp FORMATTED

...and ultra-soft pseudopotentials.
i doubt that this will conserve energy
well even for classical atom cores.
at the very least you have to crank
up the density cutoff (or dual) to
have a tight enough real space grid
to accommodate the augmentation charges.


in any case, your logic is upside down.
you _first_ have to make sure that your
simulation parameters conserve energy
withOUT thermostats and once your system
is running stable for a while, _then_
you can turn them on (and i highly 
recommend to do this in stages) to 
maintain adiabacity and the NVT ensemble.

cheers,
   axel.



-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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