fix rigid

Dear All,
I am trying to use “fix rigid” during minimization to avoid intra-molecular displacement. Here is the syntax which I am using in my input file,

fix bb all rigid group 4 gr gr1 gr2 gr3 torque * off off off

where gr, gr1, gr2,gr3 are four group of atoms (without any common atom) for which I want only rigid body displacement. However, after few steps of minimization, molecular structure starts distorting, indicating intra-molecular movements.
Am I using correct command ? if not which will be appropriate command to do this.

Thanks
Hemant

Constraints are not invoked during minimization.
You might replace constraints with large force constants. Or do NVT simulations at a low temperature instead.

Francis

Constraints are not invoked during minimization.

correction, the rigid fixes are *not* constraints but rigid particle
integrators that integrate the center of mass motion and the
rotational motion of an extended particle formed from the constituting
atoms. nevertheless, they are indeed not invoked during minimization,
same as constraints via shake.

You might replace constraints with large force constants. Or do NVT
simulations at a low temperature instead.

or rather do simulated annealing using fix nve and fix viscous.

axel.