The momentum for the generalized coordinate &Theta; of the center of mass of the simulation cell under
torsional periodic boundary condition (tPBC) or bending periodic boundary condition (bPBC).

<p>
For details, see <a href="http://www.stanford.edu/~caiwei/papers.html#36">J. Mech. Phys. Solids 56, 3242, (2008)</a>.

<p>
Also see <tt>P_com</tt>, <tt>L_com</tt>.

