[gmx-users] RE: Energy conservation in collision

Janne Hirvi janne.hirvi at joensuu.fi
Fri Apr 27 17:18:02 CEST 2007

>>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
>>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,
> 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".


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