[CPMD-list] Re: some parameters in path integral molecular dynamics

[Juerg Hutter]

Hi
>
>   I often see 7 a.u. in literature. Is there any rule of thumb for timestep?

No. Values of 3 to 7 a.u. might be possible depending on the system on the
choice of other parameters (emass,cutoff).

>
> > - change the value for NOSE IONS (10000) to a more physical value
>
>   So far as I understand, the frequency of the ionic thermostats should be somewhere
between the highest and lowest frequencies of the system.
For instance, a H2O molecule has 3 modes, 2 around 3900 cm-1, 1 around 1600 cm-1,
so a thermostat frequency of 3000 or 2000 cm-1 is reasonable.
And its exact value is not important, since it works just through anharmonicity. Am I right?

Yes, but I would place it close to the highest density of vibrational
states.

>
> > - increase the taget value of the electronic kinetic energy by at
> > least  an order of magnitude
>
>   Do you really mean "increase", 0.001 -> 0.01? Or "decrease"? What's the logic behind this?
>

A system has a 'natural' electronic kinetic energy and when using
thermostats it makes sense to put the target value close to this.
The value of 0.01 was a bold guess on my side. Have a look in the
mailing list archive. This topic was discussed recently.

> > - decrease the value for NOSE ELECTRONS to 8000 - 12000
>
>   As I know, the electronic thermostat frequency should be at least 3 times of the
highest mode of the system. Is there any drawback to go higher?

Yes, it gets more difficult to integarte the eom's

>
> > - do you really need a Nose-Hoover chain of length 10 for the ions?
>
>   Hmm ... :-) My supervisor's idea is that adding thermostats costs little CPU time,
and since a molecule is highly harmonic, it's safer to use more!
However, I note that 4 thermostats are already enough to bring a
harmonic oscillator to canonical distribution.
Does that mean 4 (the default) is enough for every case?

I would guess 4 should be enough. Don't forget you have massive
themostats, meaning there are 3*N chains of length 10, therefore
30*N DOF in your thermostats alone. Together with the more elaborate
integration techniques needed this might need quite some CPU time.

>
> > - ramp up the Suzuki/Yoshida integrator
>
>   What's the usage of this?
>

Nose thermostats are difficult to integrate and would need a much
smaller timestep if the standard Verlet algorithm would be used.
There is an elaborate multiple timestep algorithm implemented in
CPMD that allows for efficient and accurate integration
(see for example the Tuckerman-Parrinello paper from 1995).
To have a better integrator here, might allow you also to increase
the time step in the CPMD calculation.

regards

Juerg Hutter


>   Thanks a lot! Best regards,
>
>   Lan-Feng
>
> >  NOSE PARAMETER
> >  10 4 4 6. 25 4
> >
> > regards
> >
> > Juerg Hutter
> >
> >
> > On Mon, 11 Jul 2005, Lan-Feng Yuan (lyuan at Princeton.EDU) wrote:
> >
> > >   Hi, dear community,
> > >
> > >   I am practising path integral calculations in CPMD. To my
> > surprise, sometimes the dynamics diverge in several steps
> > generating "NaN" (not a number). For example:
>
> > >   I also note that "EHAM", which is supposed to be conserved to
> > machine accuracy, changed before "NaN" appears. This calculation
> > is for a H2O molecule in a 10 A cubic cell (SYMMETRY 1), cutoff
> > 100 Ry, Trotter Dimension 8, FUNCTIONAL PBE (NEWCODE), and in
> > &CPMD section:
> > >
> > > MOLECULAR DYNAMICS
> > > PATH INTEGRAL
> > > NOSE IONS MASSIVE
> > >    300.0  10000.0
> > > NOSE PARAMETERS
> > >    10  3  3  6.0  7  1
> > > NOSE ELECTRONS
> > >    0.001   20000.0
> > >
> > >   The initialization of the 8 beads is done by "DEBROGLIE
> > CENTROID 300.0" in &PIMD and "ISOLATED MOLECULE",
> > > "OPTIMIZE WAVEFUNCTION", "PATH INTEGRAL" in &CPMD. What could be
> > the reason? Thank you very much! Regards,
> > >
> > >   Lan-Feng
> > >
> > > ----------------------------
> > > Dr. Lan-Feng Yuan,
> > > Postdoctoral Associate,
> > > Department of Chemistry,
> > > Princeton University,
> > > Princeton, NJ 08544, USA
> > > Office Phone: 1-609-258-1834
> > > Fax: 1-609-258-6746
> > > ----------------------------
> > >
> > > _______________________________________________
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> > > CPMD-list at cpmd.org
> > > http://cpmd.org/mailman/listinfo/cpmd-list
> > >
> >
>
>