[gmx-users] Impact velocity

[Janne Hirvi]

Hello!

I am trying to generate center of mass velocity to a water droplet so that I
could study the effect of impact velocity in the collision with a solid
surface.

First, I simply tried to generate extra velocity and compensate the increase in
temperature by decreasing other velocity components. However at the beginning
of the collision simulation temperature increased by value which corresponds
approximately to the temperature increase due to extra velocity. This is
probably due to poor equilibrium when other velocities are scaled. Temperature
coupling will equilibriate temperature but at the same time generated center of
mass velocity decreases.

Then I tried to to accelerate a single droplet in vacuum with temperature
coupling so that I could extract "equilibrium" droplets in 300K with wanted
velocities and tranfer these droplets on the studied surfaces. However I still
have a problem with temperature. When I use g_energy, droplet temperature is
wanted 300K but g_traj informs that temperature is much higher when I take into
account constrained freedoms by multiplying with 1.5 (rigid SPC). Temperature
given by g_traj actually equals to temperature given by g_energy when I
transfer the droplet on a surface for collision simulation and remove
accelaration. 

I am not sure what is happening and would be pleased if I get some clarification
and maybe other suggestions how to generate center of mass velocity for
specific temperature.

Thanks,

Janne