[CPMD-list] Energy components in PIMD

[Łukasz Walewski]

Dnia 2008-05-14, śro o godzinie 12:35 -0400, Axel Kohlmeyer pisze:
> 
> dear lukasz,

Dear Axel,

> first off, let me commend you for being so careful 
> and persistent. this is giving a great example to
> other people and it has been very educational to me
> (and i hope for the many other "lurkers" on this list
> as well).

Good to hear that. Thanks!

> LW> > since the machine i have available for these kinds of tests, 
> LW> > i cannot run that long. please see the first attached graph. 
> LW> > it does show a little slant now.
> LW> 
> LW> Going down to 200 a.u. with e-mass seems to stabilize the drift in EKINC
> LW> more or less (with the timestep of 3 a.u) - see attached graph. Do you
> LW> think it's reasonable ? Would it be sane to go even to 150 a.u. and
> LW> possibly reducing the time step to 2 a.u. (having the future PIMD
> LW> simulations in mind) ? 
> 
> hard to tell. your system is definitely a bit more sensitive
> that the typical "simple-bulk-where-many-errors-cancel-out"
> setup that people prefer to study for convergence analysis...
> 
> one thing that came to my mind, is that you may improve the
> situation by turning the "outside" hydrogens into deuterium.
> their bonded vibrations seem to be of pretty high frequency and 
> thus slowing them down by increasing their mass may help improving 
> the stability w.r.t. timestep and ficitious mass.

The energy gap btw. HOMO and LUMO in my system is 1.3 eV ~= 30 kcal/mol.
I don't have much experience with other systems but e.g. in water
molecule this value is around 6 eV. The small gap may result in small
lowest frequency in electronic power spectrum. This in turn may coincide
with high frequencies of the protons motions and thus the coupling...
>From this perspective turning protons into deuterons (which lowers the
ionic frequencies) seems to be in the right direction.

> LW> > BTW: based on its "spectrum" i have changed coupling frequency
> LW> > for the N-H chains to 2500 cm-1, if you look at the second 
> LW> > attached graph this seems a much more reasonable choice as it
> LW> 
> LW> What do you call "spectrum" in this case ? - with FFT(EKINC) I get
> LW> something different...
> 
> this was run though the fourier.x code and "temperature corrected".

Hmm... fourier.x does not seem to be bundled with CPMD (nor CP2K). Where
can I find it (Google doesn't know) ?

> you could generate the same with something like:
> 
> awk '{print $1, $2, 0.0, 0.0, $2, 0.0, 0.0}' ENERGIES > out
> 
> (echo out ; echo spec.dat ; echo 3.0 ; echo 300.0 ; echo 20000.0 ; echo 1 ) | fourier.x
> 
> and then plot columns 1 and 5 (columns 1 and 2 are what you probably see).

>From the above this is not clear to me what is the content of spec.dat ?

This wouldn't be worth that much noise if only the "temperature
correction" didn't sound that mysteriously. See the other thread ("Power
spectrum ...") on this list.

> BTW: i made a test with BO dynamics. when using wavefunction 
> extrapolation and some other tricks (that are not yet in 3.11.)
> i get very good energy conservation with the attached input,

The input I received (as well as the previous one in this thread)
contains some lines at the beginning of the file that do not belong to
any &SECTION ... &END section. Is it intentional ?

[...]

> cheers,
>    axel.
> 
> p.s.: after going through so many tests and checking out so many 
> details. what do you think about summarizing your experiences and
> adding it to the CPMD manual as a guide for other CPMD users?

I would avoid making any promises here but I will keep this in mind.

Best regards,
Lukasz