Extended structures nonbonded interactions

Geometry errors increase with the size of a system when the number of nonbonded interactions increases, and folded conformations are differently affected than extended ones. That is, MP2-energies calculated at HF-geometries can be inaccurate, because geometry errors are not necessarily the same in different conformations of the same molecule. Regarding the molecular structures it can be shown that, even when differences between MP2 geometries and the HF equivalents are small, they can be significant. 

A supramolecular but nonmolecular lattice might be considered to be an oxymoron. However, as mentioned earlier, there is a considerable number of crystals which contain interpenetrating but unconnected infinite lattices. In the context of supramolecularity, the special property of these crystals is that there are two or more sets of atoms which are connected within the sets but not between the sets (just as in molecular structures), and there are extended nonbonding boundaries (as in molecular lattices) but there are no finite molecules. The crystals are maintained by the interactions across non-bonding boundaries. 

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