Defines the center of rotation for the molecule to be the geometric center of the selected molecules.
If no atoms are selected the the center is reset to the center of the bounding box defined by the extreme atom centerpoints in each of the x, y, and z dimensions. This is the default.
If one atom is selected then it becomes the center of rotation.
If two or more atoms are selected then the center becomes the point defined by the mean of the x, y, and z coordinates.
{If you think there is a better definition for this (center of gravity?) then let us know and we can either change this or add a new definition.}