Natural Lewis structure accuracy

Let us first inquire whether basic criteria for the validity of low-order perturbation theory are actually satisfied in the present case. As described in Section 1.4, the perturbative starting point is an idealized natural Lewis-structure wavefunction (t//,l )) of doubly occupied NBOs. The accuracy of this Lewis-type starting point may be assessed in terms of the percentage accuracy of the variational energy (E) or density (p(l ). as shown for each molecule in Table 3.20. 

It must be emphasized that the formulation of three-center T-bonds provides a qualitative (not merely incremental) improvement in the accuracy of the natural Lewis-structure description of diborane. Because a three-center orbital is intrinsically more mathematically flexible than a two-center orbital, the description of any molecule is seemingly improved by employing three-center in place of two-center NBOs. However, for most non-boron molecules this improvement would be quite negligible (e.g., less than 0.1% for ethane, whose two-center Lewis-structure... 

Table 4.36. The NBO descriptors o/ M2H2 species (see Fig. 4.57) namely the accuracy of the natural Lewis structure (%/+). metal charge (Qu), metal hybrid (hM), percentage polarization toward metal (%m), and occupancies (Occ.) of two-center bonds and antibonds...

Table 11.2 Calculated bond length (Re), NRT bond order ( no) atomic charge at N (2n) and a/p natural lewis structure (NLS) representations (and percentage accuracy) for low-lying states of nitric oxide (CIS/6-311 +G level) (cf. Fig. 11.1a for state labels).

Table 3.29. The NBO descriptors of XYZ triatomic anions (see Table 3.28), showing natural atomic charges (Q), percentage polarization of oxy and ayz NBOs toward terminal atoms, second-order nz— oxy and nx->OYZ stabilizations (A/s(2)), and occupancies of valence antibonds (oxy and ayz ) and extra-valent Rydberg orbitals (ry ) parenthesized values refer to Lewis structure of lower accuracy...

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