damp parameter in”fix langevin command”

2011/4/6 XiaoJing <[email protected]>:

Dear all,

     First thank you for good suggestions, I have solved the warning
"Particle deposition was unsuccessful".

     I have been confused about the damp parameter in"fix langevin command",
when I used smaller damp parameter, atoms on surface form cluster, when I
use lager damp parameter, atoms on surface did not form cluster. I wonder
how to understand this? The diffusion of atoms on surface is determined by
the interaction between surface and atoms. Now the results show that
different damp parameter can affect the behavior of the atom cluster, How to
connect the dump parameter with the physical properties? I see in manual, it
may correspond to viscosity, but to specific material under same
temperature, does the viscosity is a constant? Why it can be altered by dump
parameter? To the growth issue, how to understand the physical meaning of
the influence of damp parameter on the state of atoms on surface?

have a look at the langevin thermostat formula and its derivation.
it is a "dissipative" thermostat, i.e. meant for condensed systems
or for system with implicit solvent. the temperature control is done
via a balance between a friction term (which dampens high frequencies
more than low) and a random term (which helps to evenly distribute the
kinetic energy through the system). the smaller the damp factor, the
larger these two terms are and as a consequence, free motion is
suppressed in favor of a random walk.

when you do a non-equilibrium system like deposition, you usually
only want to thermalize a lower part of the slab that you deposit on,
have the bottom layer fixed and the rest integrated with fix nve.


Thank you!
I have fixed the substrate, and use "fix nve" combining "fix langevin" to
control Temperature. If I only use "fix nve", how do I control temperature
of the upper-region near surface?