NMR-K reduced coupling constants within the CLOPPA-PM3 approach. II: shortcomings and how to overcome them

The shortcomings encountered in the CLOPPA-PM3-RPA coupling constant calculations shown in part I of this work are analyzed. These are: (i) the Hartree-Fock quasi-instabilities of the non-singlet type in PM3 wavefunctions; and (ii) the erroneous PM3-derived description of the hybridization of some localized molecular orbitals. It is shown that these problems arise from: (a) the parameter's optimization procedure used within the PM3 scheme, which apparently takes into account some effects at the nuclear sites in an inappropriate way; and (b) the selected set of properties chosen to be reproduced by the parameter's optimization procedure. These properties do not critically depend on the electronic surroundings of the nuclei. Hence, some effects at nuclear sites are badly reproduced when the PM3 wavefunction is used. We have found three parameters which are mainly involved: the one-center two-electron integrals; the beta(s) or beta(p) parameters; and the U-ss one-electron energies. The FC electronic mechanism is the most affected by the problems analyzed here. (C) 1998 Elsevier Science B.V. All rights reserved.