The role of second-order perturbations in interlayer interactions of two-dimensional electronic systems is examined from simple two-layer models. The preferred nuclear motions are discussed specifically for the preferred stacking of layers in graphite on the basis of transition-density analyses for extended systems. The well-known ABAB stacking of layers in neutral graphite and the AAAA stacking of layers in graphite intercalation compounds (GICs) such as C6Li and C8K complexes are proposed to be consequences of orbital interactions (near the Fermi level) between nearest neighbor layers. This paper raises doubts about a widely accepted belief that the interlayer interaction between graphite layers is of the van der Waals type.
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