Hi Andrew,
I think that thermostatting a quantum system at very low temperature is not easy and requires quite a lot of equilibration time. It is also very likely that a Nose-Hoover chain with 4 thermostats is not enough for such low temperatures so you may want to increase the number of thermostats (nchain). At the moment, the maximum number is set to be 9 but you can change it and make it larger in nose_hoover.f.
Best wishes,
Francesco
----------------------------------------------------------------------
Francesco Paesani, Ph.D.
Assistant Professor
Department of Chemistry & Biochemistry
University of California, San Diego
Urey Hall 3242
9500 Gilman Drive #0314
La Jolla, CA 92093-0314
phone: (858) 822 - 3383
email: fpaesani.ucsd.edu
web: http://paesanigroup.ucsd.edu
----------------------------------------------------------------------
On Aug 23, 2011, at 7:19 AM, Andrew Petersen wrote:
> Francesco
>
> I think I figured out the problem. For <64 beads, the temperature of the
> system is tightly regulated when there is a change in temperature, for
> example starting with 0.1K (tempi=0.1 or restarting from a 0.1K simulation)
> and ending with 5K (temp0=5K).
> However for 64 beads and more, the temperature oscillates widely if the
> restart and target temperatures are not close.
>
> Andrew
>
>
> On Sat, Aug 13, 2011 at 7:07 PM, Francesco Paesani <fpaesani.ucsd.edu>wrote:
>
>> Hi Andrew,
>>
>> I believe that it is normal. You need to find the right parameters for
>> running NMPIMD with the number of beads that you need. It is quite possible
>> that a timestep of 0.00001 is not enough. Everything depends on the
>> interactions, atom masses, temperature and number of beads. Please look at
>> the references about PIMD in general to see how the frequencies of the
>> springs that connect the beads depends on these quantities. From that
>> analysis you should be able to determine the right timestep for your
>> simulations.
>>
>> Best wishes,
>> Francesco
>>
>> ----------------------------------------------------------------------
>> Francesco Paesani, Ph.D.
>> Assistant Professor
>>
>> Department of Chemistry & Biochemistry
>> University of California, San Diego
>> Urey Hall 3242
>> 9500 Gilman Drive #0314
>> La Jolla, CA 92093-0314
>> phone: (858) 822 - 3383
>> email: fpaesani.ucsd.edu
>> web: http://paesanigroup.ucsd.edu
>> ----------------------------------------------------------------------
>>
>>
>>
>>
>>
>>
>> On Aug 13, 2011, at 8:11 AM, Andrew Petersen wrote:
>>
>>> Hello users,
>>>
>>> Im running nmpimd for a small system. 4,8,16,32 beads works fine. But
>>> 64beads, there is slow heating. With 128beads the heating is rapid and
>> the
>>> simulation crashes. Reducing the step size from dt=0.0001 to 0.00001
>> does
>>> not help. I would appreciate any help I can get to fix this.
>>>
>>> Regards
>>> Andrew
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>>
>>
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Received on Thu Aug 25 2011 - 09:00:02 PDT
