[lammps-users] fix-rigid, thermostatting and barostatting

hi all,

I am trying to do molecular dynamics on a collection of rigid molecules, using 7 July 2009 version of LAMMPS. The molecules are composed of charged point masses and the intra-molecular interactions are turned off. Inter-molecular interactions are computed as a sum of Buckingham potential and long range Coloumbic interactions. I have a few concerns which i am listing below.

  1. I am using fix rigid to update the positions, velocities and orientation of the rigid bodies in my simulation. What algorithm does LAMMPS use to update the orientations? (Evans quaternion or similar?)

  2. According to the doc files, the way to control the temperature of such a system is through a Langevin thermostat. But the doc page for the fix_langevin explicitly states that this fix modifies only translational velocities. I understand that one can modify the temperature compute used by this fix to account for both rotational and translational KE. Is it possible to modify this fix to scale both the translational and rotational velocities?

  3. Trying to barostat this sytem using a fix_berendsen results in undesirable results (blows up). Looking in the mailing lists, the latest post ( http://sourceforge.net/mailarchive/message.php?msg_id=33ea1b0a0909140717k1f06104ah93f6ce54cb5861dd%40mail.gmail.com) suggests new capabilities being added.