[gmx-users] "Angular" Vs. "Linear" in NVE simulation

[Antonio Baptista]

On Fri, 4 Nov 2005, David wrote:

> On Fri, 2005-11-04 at 23:32 +0800, Xiaobing Feng wrote:
>> Dear everyone,
>>
>>     When I run a NVE simulation with "Angular" the
>> total energy decreased very rapidly, though the
>> system is equilibrated with NPT. Once "Angular" was
>> replaced by "Linear" the total energy is well
>> conserved.
>>
>>    "Angular" removes the drifts of both center-of-mass
>> and angular, so, it seems better to use "Angular".
>>
>>    Could anybody give me an explanation why "Angular"
>> is bad in NVE?
>
> Not sure. It could be a precision problem, but I think we have tried it
> in double as well. Have you tried that?

If you are using periodic boundary conditions (pbc) in your NVE 
simulation, angular momentum _should_ not be conserved, regardless of the 
precison you use.  Conserved quantities are a consequence of symmetry 
properties: linear momentum is conserved when space is homogeneous 
(invariant under translation); angular momentum is conserved when space is 
isotropic (invariant under rotation). Now, the use of pbc does not mess 
you the homogenity of space, but kills its isotropy because it does not 
have spherical symmetry. When you don't use pbc (eg, when simulating a 
cluster) the space is sperically symmetric and angular momentum _should_ 
be conserved.

This issue is mentioned in most simulation textbooks (Allen & Tildesley,
Leach, etc). If you are curious about these symmetry matters you can have
a look at the Feynman Lectures for a nice overview, or even at Goldstein
(classical) or Messiah (quantum) for the heavy stuff.

Regards,
Antonio

>
>>
>>     Many thanks in advance
>>
>>       Xiaobing
>>
>>
>>
>>
>>
>>
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> -- 
> David.
> ________________________________________________________________________
> David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
> Dept. of Cell and Molecular Biology, Uppsala University.
> Husargatan 3, Box 596,          75124 Uppsala, Sweden
> phone:  46 18 471 4205          fax: 46 18 511 755
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--
Antonio M. Baptista
Instituto de Tecnologia Quimica e Biologica, Universidade Nova de Lisboa
Av. da Republica, EAN, ITQB II, Apartado 127
2781-901 Oeiras, Portugal
phone: +351-214469619         email: baptista at itqb.unl.pt
fax:   +351-214411277         WWW:   http://www.itqb.unl.pt/~baptista
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