The most stable packing scheme of non-charged rigid molecules must fulfill two conditions:

1. Fraction of occupied space is maximized, AND the change of this fraction upon pertubation movement of the molecules is minimized.

2. Contact area between atoms is maximized, AND the change of this contact area upon pertubation movement of the molecules is minimized.

An energy function can be constructed based on this concept and the metastability of any packing scheme can be measured with this energy function.