The forces and torques on the COM are rotated into the local molecular frame using the rotation matrix defined in section (2) in the main In this frame the 5 known COM forces -the three components of F t 0 and the two non-zero components of ⌧ t 0 -are used to solve for the 9 unknown atomic forces. This set of equations is undefined, hence additionally, and without loss of generality, the following atomic force components are set to zero: