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Re: [ESPResSo-users] Thermostat, Rigid bond & Rotation feature

From: Axel Arnold
Subject: Re: [ESPResSo-users] Thermostat, Rigid bond & Rotation feature
Date: Thu, 2 Jun 2011 14:29:14 +0200
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On Wednesday 01 June 2011, Muhammad Anwar wrote:
> Hello,
> Dear Users & Developers,
> I am using espresso-2.2.0b and working on simulation of polymer melt,
> first i simulated a system with Lennard Jones particle without any
> bonded potential at constant temperature (I put temp 1) using Langevin
> thermostat. I calculated the temperature which fluctuate around 1 (0.99
> - 1.02) and total pressure is in three digits around 130. Then i added
> rigid bond my temperature fluctuate from 0.94 to 0.98 and total pressure
> increases to 6 digits around 490000.

Since you don't provide your script, we can only guess. However, the pressure 
presumably goes up since some particles are constrained at distances with high 
interaction; you might want to check the pressure components to see which 
interaction that is. You might want to exclude intramolecular nonbonded 
interactions, since they are meaningless in case of rigid bonds.

> Then i added cos square bond angle
> potential, my temperature fluctuate from 0.64 to 0.69 and pressure again
> in 6 digits. Then i added dihedral potential as well and again
> temperature  fluctuate from 0.64 to 0.69 and pressure in 6 digits. I
> kept epsilon, sigma and other parameters in Lennard Jones and number of
> particles same.

If the pressure is too high, probably also some interaction is quite large, 
which might make it impossible for the thermostat to keep the temperature. 
Check with a smaller time step, e.g.

> I want to ask what reason might be for smaller value of temperature when
> adding bonded potentials and why pressure increases to a big value when
> add rigid bonds, does it make sense?

See above.

> Secondly, when i activate ROTATION feature along with rigid bond, i get
> the message , currently i can not activate bond constraint and rotation
> feature together.

As it says, you cannot use ROTATION with rigid bonds. The reason is that there 
is no constraints algorithm for rotationally non-invariant particles. If you 
find a good one, you can try to implement it and contribute to Espresso.


JP Dr. Axel Arnold      Tel: +49 711 685 67609
ICP, Universit├Ąt Stuttgart      Email: address@hidden
Pfaffenwaldring 27
70569 Stuttgart, Germany

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