[lammps-users] fix langevin and fix rigid

Hi, guys,

While using fix rigid, I got some questions.

My system is composed of two molecules, lets say group_A and group_B. Important fixes are like below,

pair_sytle lj/cut/coul/long 10.0

fix 1 group_A rigid

fix 2 group_A langevin 300.0 300.0 100.0 34958

fix 3 group_B nvt 300.0 300.0 100.0

run 1000

Here’s my question.

group_B à for nvt time integration, total forces applied to group_B molecules are sum of pairwise(vdw, coulomb…) and molecular(bond, angle…) potential.

group_A à time integration is performed via fix rigid (nve). Then, where do the forces come from for nve integration? Fix langevin also calculates forces (= sum of frictional drag, force due to solvent atoms bumping into particle and inter-particle interaction force). The Lammps manual says “fix langevin only modifies forces to affect thermostatting. Thus you must use a separate time integration fix, like fix nve to actually update the velocities and positions of atoms using the modified forces” à which means nve integration uses “modified forces” by fix langevin.

According to above information, group_A molecules use forces obtained by fix langevin which does NOT include pairwise (vdw, coulomb…) potential interacting with group_B. Is it right?

Atoms in group B, whether they are part of molecules
or not, are acted on by each other, and by group A atoms.
The total force on each atom is integreted in your NVT fix.

Atoms in group A are acted on by each other, by group B
atoms and each atom is given an additional (randomized)
force by your fix langevin. Because A is treated as
a rigid body and integrated by fix rigid, the A-A forces
cancel out. But the forces from B atoms and fix langevin
contribute to the motion/rotation of the rigid A body.