CHOOSE keylist

The occupied orbitals on the left in Equation 7.13 are both Lewis-type (unstarred), but on the right is non-Lewis-type, and its occupancy is not accoimted to PLe. In rare cases where the standard NBO search leads to such a hinh-occupancy and pair, the error associated with occupancy is not properly taken into account, and the search may return a structure that is not the best possible in the maximum-occupancy sense (as can be checked with a CHOOSE keylist). This type of anomaly cannot occur when (rather than PLewis) is chosen as the variational objective function. For this reason we recommend that the generalized EXCITE search in NBO 5.X be used in all cases, including ground states, even though it requires additional computation time and usually gives the same result as NBO 5.0. 

Alternately, we can examine the same features of the IRC from the product point of view, using a CHOOSE keylist to specify product-like NBOs throughout the reactive region (to continue the trends seen at the right edge of Fig. 10.6). As shown in Fig. 10.6, the two principal product-type NBO delocalizations... 

The accuracy of alternative Lewis structures can be assessed by specifying the number and locations of lone pairs and two- and three-center bonds with a SCHOOSE keylist (a standard option of the NBO program). (The directed SCHOOSE list replaces the usual NBO search over all possible Lewis structures, but the hybrids and polarization coefficients of CHOOSE structures are optimized in the usual way.) The increased non-Lewis density measures the larger error of the CHOOSE structure compared with the optimal NBO structure. 

The NBO program also permits an alternative quasi-vari-ational deletions ( DEL keylist) method to evaluate the effects of specific NBO donor-acceptor interactions. In effect, one can choose to delete any particular interaction (or a particular non-Lewis orbital, etc.) and recalculate the variational energy and other properties as though these interactions or orbitals were absent in nature. In this approach, removal of a particular CD delocalizing interaction raises the variational... 

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