[gmx-users] Energy conservation in collision

[Berk Hess]

>From: Janne Hirvi <janne.hirvi at joensuu.fi>
>Reply-To: Discussion list for GROMACS users <gmx-users at gromacs.org>
>To: gmx-users at gromacs.org
>Subject: [gmx-users] Energy conservation in collision
>Date: Thu, 26 Apr 2007 17:36:29 +0300
>
>Hello gmx-users!
>
>I am still struggling with the energy conservation in the system where a 
>water
>droplet (consisting of rigid molecules) collides with a frozen structured
>surface. The total energy is well conserved at the beginning of NVE 
>simulation
>when the droplet is approaching the surface and once again when the droplet 
>is
>stabilized on the surface, but the drift or fluctuation in the total energy 
>is
>observed between these extremes.
>
>The total energy of the system (~300 000kJ/mol) decreases slightly 
>(<100kJ/mol)
>at the collision and on non-wetting surfaces (water droplet wont intrude to 
>the
>pores of the surface) part of the energy reverts quickly causing a sharp 
>peak
>in the energy curve. This could indicate an integration error at the 
>collision
>due to the large time step, but equal behavior is observed with the time 
>steps
>of 2.0fs and 0.5fs.
>
>On the wetting surfaces (water droplet intrude to the pores of the surface) 
>the
>center of mass motion of the droplet come to a stop at the collision as in 
>the
>case of non-wetting surface and sharp decrease in the total energy is 
>observed.
>However, now none of this energy is reverted and the total energy stabilize 
>to
>this value until the water molecules starts to intrude to the pores of the
>surface. The total energy decrease slowly in conjuction with the slow 
>sinking
>of the height of the center of mass of the water droplet until the bottom 
>of
>the pores is reached. After the bottom of the pores is reached energy will
>stabilize to the corresponding value which is about 200-300kJ/mol lower 
>than at
>the beginning.
>
>Decreasing the time step from 2.0fs to 0.5fs have no influence for the 
>energy
>conservation and somehow the problem seems to be connected to water-surface
>interactions: on the wetting surfaces (more water-surface interactions) a 
>drift
>is observed instead of a sharp peak of non-wetting surfaces (less 
>water-surface
>interactions). Any ideas how I could achieve complete energy conservation?
>
>Thanks for your time and help,
>
>Janne

What kind of interactions are you using?
Plain cut-off for Coulomb and LJ will give bad energy conservation.
For your system you probably want to use shifted LJ and Coulomb potentials.

Berk.

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