[gmx-users] RE: 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] RE: Energy conservation in collision
>Date: Fri, 27 Apr 2007 18:18:02 +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 atthe 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.
>
>I am using PME with slab correction for Coulombic interactions and plain 
>cut-off
>for Lennard-Jones interactions. Neighbor lists are updated every time step 
>due
>to the use of plain cut-off for LJ interactions. This actually gives really
>good energy conservation at the beginning of the simulation, when the 
>droplet
>is approaching the surface and once again when the droplet is stabilized on 
>the
>surface, but the problems exist between these "stable situations".

You don't specify how you made your surface,
but I assume it would be with LJ particles.
Then you have jumps in the energy with a cut-off, since the LJ potentials
is not zero at the cut-off. This will result in lower energies at contact.

Use a shifted potential, in that way you can also set nstlist to something 
like 10
which will speed up your simulations.

Berk.

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