[CPMD-list] Energy components in PIMD

[Łukasz Walewski]

Dnia 2008-05-12, pon o godzinie 10:10 -0400, Axel Kohlmeyer pisze:

Dear Axel,

[...]

> BTW: based on its "spectrum" i have changed coupling frequency
> for the N-H chains to 2500 cm-1, if you look at the second 
> attached graph this seems a much more reasonable choice as it
> couples to a wider selection of modes and i particularly don't 
> think it is a good idea to mess with the mode that interests
> you directly. it will of course be excited because its overtones
> are included in the frequencies here.
> 
> LW> When I reduce the electron mass to 300 a.u. the drift is slower
> LW> (http://bioexploratorium.pl/ljw/cpmd/nvt/axel/EKINC-1.png) but reducing
> LW> e-mass seems to be a poor strategy since from some point I would have to
> 
> when you turn on path-integrals, you _will_ have to reduce the time step, 
> anyways (unless you run BO-dynamics).
> 
> LW> decrease also the time step which cancels the effect. It also seems to
> LW> me that it is impossible to achieve exact adiabaticity for my system.
> 
> yes. it looks like your system is a somewhat pathological case.

I think I tracked down the problem. The power spectra (that I finally
managed to reproduce - thanks for the fourier.x code) overlap with the
ionic vibrational frequencies! The highest vibrational mode is 3190 cm-1
while the EKINC spectrum starts around 1000 cm-1 (see the first attached
graph). I will try to reduce the hydrogen oscillations by deutering the
outer protons with the hope that this will not perturb the inner proton
transfer - will I have to correct for kinetic isotope effects in this
case ? (No, I guess.)

However I have two more observations:

1) Reducing the electron mass while keeping the ionic masses unchanged
reduces the EKINC drift anyway - see the second graph. This must happen
regardless of the apparent coupling btw. electronic and ionic DOF.
Moreover switching from MT to VDB pseudopotentials and reducing the
r-cutoff from 70 to 35 Ry while keeping the other parameters fixed does
actually improve the adiabaticity (cf. green and blue plot on the second
attached graph).

2) The power spectra of electronic DOF do not change much upon change of
the electron mass - on the first attached graph there are in fact two
plots: MT, 70 Ry, mu=400 a.u., dt=3 a.u. (black) and VDB, 35 Ry, mu=200,
dt=3 a.u. (red). The two spectra are almost identical.

Trying to join those two observations I conclude that there must be some
mechanism, that prevents energy transfer from ionic subsystem to
the electronic one, concurrent to the frequency coupling. What can it
be ?

Thank you guys for the patience...

Lukasz
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