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

[Lan-Feng Yuan (lyuan@Princeton.EDU)]

----- Original Message -----
From: Juerg Hutter <hutter at pci.unizh.ch>
Date: Tuesday, July 12, 2005 4:45 am
Subject: Re: [CPMD-list] "NaN" in path integral molecular dynamics?

  Dear Juerg and people,

  Thank you very much for your suggestions. Now with timestep 4 a.u. I can make the PIMD run smoothly. Here I want to address some more questions about parameter setting in PIMD, and would deeply appreciate any comment. 
> 
> your calculation is obviously non-stable. There is an onset of 
> instabilitystarting at step 6 that leads within a couple of 
> addtional step to a full
> divergence. Reasons for this could be a problem with equilibration,
> meaning you have to choose system parameters that allow for more
> gentle approach to equilibrium.
> I would try the following changes
> 
> - reduce the timestep

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

> - 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?

> - 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?

> - 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?

> - 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?

> - ramp up the Suzuki/Yoshida integrator

  What's the usage of this?

  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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>