Page 134 - Valence Bond Methods. Theory and Applications

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8.3 Covalent and ionic curvð crossingiin LiF
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shows,occur between the Li −F statŁ curve and E 3 and E 4 . These latter two
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states involve Li2p σ –F2p σ and Li2p π –F2p π ,botà coupled to . The crossing
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in these cases occur at sucà largŁ distances therŁ is verł littlŁ interaction.
The calculation in this illustration werŁ not donŁ wità a minimal basis set,since,
if sucà werŁ used,they would not show the correct behavior,even qualitatcvelł.
This happen because wŁ must represent botà F and Fin the samŁ wave function.
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Clearlł onŁ set of AOs cannot represent botà states of F. Li does not present sucà
a difficult problem,since,to a first approximation,it ha either onŁ orbital or
none. The calculation of Fig. 8.4 werŁ donŁ wità wave function of 1886 standard
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tableaux functions. These support 1020 symmetrł functions. We will discuss
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the arrangement of bases morŁ fullł in Chapter 9.