espressomd-users
[Top][All Lists]

## Re: [ESPResSo-users] VIRTUAL_SITES

 From: Rudolf Weeber Subject: Re: [ESPResSo-users] VIRTUAL_SITES Date: Thu, 21 Aug 2014 17:06:57 +0200 User-agent: Mutt/1.5.21 (2010-09-15)

```Hi,
On Thu, Aug 21, 2014 at 04:50:33PM +0200, Dudo wrote:
> please, I would need a sort of under the hood information,
> do the virtual sites have mass and gamma or only if VIRTUAL_THERMOSTAT is
> called?
The mass property of virtual sites is ignored, as they are not propagated using
Newton's eqn of motion.
Rather, their position is calculated from the position and orientation of the
particle, they are related to, i.e., the particle in the center of mass of the
rigid body.
Unless you set the switch mentioned above, virtual sites are not thermostated.
Instead, the thermostating is done through the rotational and translational
degree of freedom of the particle in the center of mass of the rigid body.
If you want correct dynamics, you will probably need to set
mass of the particle in the rigid body's center of mass =total mass of all
particles forming the rigid body.
Interial tensor fo the particle in the center of mass = Inertial tensor of the
rigid body as a whole.
Here, it is important to make sure that the orthogonal basis in which you
specify the inertial tensor co-insides with the co-rotating frame of reference
of the particle in the center of mass.
The easiest way to achieve this, is to set the quaternions of the central
particle to 1 0 0 0.
In this case, the co-rotating frame of reference is equal to the lab frame.

If you only need static properties, you can ignore the mass and  inertia of the
particle in the center of mass.

It is currently not possible to set the gamma parameter of the Langevin
thermostat independently for the rotational and translational degree of freedom.

Hope that helps.
Regards, Rudolf

```