Abstract
Simple molecular orbital (MO) arguments as well as valence bond analysis of accurate MO–CI wave functions discriminate between essentially neutral and essentially ionic eigenstates, which also appear clearly from careful studies of the Hubbard bands. While homoatomic molecules have a neutral ground state, Lin clusters for n=4–8 have ionic states in the sense of the VB theory, with large static or dynamic charge displacements. All these structures involve odd-membered rings. Secondary minima of neutral VB character appear to be built from four-membered rings and have large components on spin waves. This analysis supports a qualitative difference between chemical bonding in nonpolar systems with normal valency of the atoms and the metallic bonding (involving hypervalent atoms), which cannot be regarded as resonating covalent bonds. The role of the β/U ratio (i.e., delocalization/correlation) and of dimensionality is discussed (one-dimensional clusters would remain neutral).
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