[gmx-users] Energy conservation in collision

[Janne Hirvi]

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