# [gmx-users] Impact velocity

Robert Johnson bobjohnson1981 at gmail.com
Fri Feb 16 16:14:36 CET 2007

```If you wanted to study a collision, why do it at constant temperature?
Like David said, maybe you should equilibrate the droplet at 300K, add
a center of mass velocity to the droplet, and then study the collision
at constant energy. The real collision probably doesn't take place at
constant temperature anyway. The surface probably heats up a little
upon colliding with the water droplet. Anyway, Just my thoughts...
Bob

On 2/16/07, Janne Hirvi <janne.hirvi at joensuu.fi> wrote:
> 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
>
>
>
>
>
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