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
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?