Improved documentation on CMMotionRemover (#5219)

docs-source/usersguide/theory/02_standard_forces.rst

@@ -913,15 +913,17 @@ removing all center of mass motion.  At the start of every *n*\ ’th time step
 (where *n* is set by the user), it calculates the total center of mass
 velocity of the system:
 
-
 .. math::
    \mathbf{v}_\text{CM}=\frac{\sum _{i}{m}_{i}\mathbf{v}_{i}}{\sum _{i}{m}_{i}}
 
-
 where :math:`m_i` and :math:`\mathbf{v}_i` are the mass and velocity of particle
 \ *i*\ .  It then subtracts :math:`\mathbf{v}_\text{CM}` from the velocity of every
 particle.
 
+CMMotionRemover is an effective way to prevent the system from drifting, but it is
+not a rigorous constraint.  The center of mass may still move slightly, and may
+still be observed to have a small nonzero velocity.
+
 RMSDForce
 *********
 

openmmapi/include/openmm/CMMotionRemover.h

@@ -37,8 +37,11 @@
 namespace OpenMM {
 
 /**
- * This class prevents the center of mass of a System from drifting.  At each time step, it calculates the
+ * This class prevents the center of mass of a System from drifting.  It periodically calculates the
  * center of mass momentum, then adjusts the individual particle velocities to make it zero.
+ *
+ * CMMotionRemover is an effective way to prevent the system from drifting, but it is not a rigorous constraint.
+ * The center of mass may still move slightly, and may still be observed to have a small nonzero velocity.
  */
 
 class OPENMM_EXPORT CMMotionRemover : public Force {