[gmx-users] Heat transfer in collision

[Janne Hirvi]

Hello gmx-users!

I am trying to simulate the effect of impact velocity of a water droplet in
collision with a surface. Translational energy of the water droplet is expected
to turn into the heat of the droplet and the surface in collision when NVE
ensemble is used (as in real world). 

Earlier simulations without impact velocity employed frozen surfaces which wont
allow heat transfer to the surface. Now, with non-zero impact velocity heat
transfer to the surface might however be crusial and as a first approximation I
have tried position restraints for all surface atoms:

1) If surface temperature is 0 K before collision slight decrease in the droplet
temperature is observed when compared with the case of the frozen surface due
to the warming of the surface. 

2) However, when surface atoms are coupled to 300 K with Berendsen thermostat,
droplet temperature is even slightly higher than in the case of the frozen
surface. 

I excpected to see heat transfer to surface even when surface is kept in 300 K.
Can anyone explain why heat transfer is not observed?

3) Probably the best option would be a surface which is separately equilibriated
to 300 K so that no temperature coupling is needed. I enabled to equilibriate
surface under position restraints without surface-surface interactions
(cut-offs and exclusions). Atoms vibrate nicely on an average at 300 K around
their initial positions. My original system however employes PME for Coulombic
interactions and long-range surface-surface Coulombic interactions (reciprocal
space) cant be excluded. This seems to introduce problem for equilibriation.
Under temperature coupling, fluctuations in temperature can be reduced to less
than 1 K but temperature fluctuations increase drastically (almost explode)
instantly when temperature coupling is removed.

Should I still try to get better equilibriation with NVT before NVE simulation
or is it simply impossible to equilibriate this kind of system under position
restraints? Have I done something stupid? Any suggestions for how to proceed?


Thanks,

Janne