Pseudoatoms of the electron density

Abstract

The theory of molecular structure, developed by Bader and co-workers, has met with considerable success and is now being applied to many aspects of chemistry. One finding, however, that has complicated the theory is that of non-nuclear attractors, that is, pseudoatoms. In this study we examine basis set and to some extent electron correlation effects on the appearance and disappearance of non-nuclear attractors in Li 2 , Na2' Na4, and Nas. The appearance of a pseudoatom in a lithium molecule correlates remarkably well with the size of the region, in an atomic calculation, of -V 2 p(r) for the valence shell of the atom. This and the fact that the pseudoatom is also present in the promolecule indicates that the pseudoatoms are remnants of or, in fact, are portions of atoms that are not perturbed enough in the molt~cule to remove an essentially atomic characteristic.

On the electronic structure of Li2 (X1) and its changes with internuclear distance

“…However, Hartree-Fock calculations carried out with an optimized Slater basis set (OBS-SCF) to check these ideas gave inconclusive results, since only z s at 0.735 a 0 was definitely on the low side of the SCF values reported for the largest Gaussian basis sets used in each of those references (0.735 a 0 [19]; 0.748 a 0 [22]; 0.775 a 0 [23]). No such clear and uniform trend appeared for the values of ρ and its derivatives at either critical point.…”

“…The OBS-GMCSC values of the three-dimensional electron density Laplacian, ∇ from the CISD results in Glaser et al [22] (individual eigenvalues of the electron density's Hessian not given by Edgecombe et al [23]). …”

“…The location of at least one of the two bifurcation points has been shown to be significantly dependent on the inclusion of electron correlation [21] and on the basis set employed [24]. On the other hand, a suggestion that the existence of a range of internuclear distances over which the electron density displays two non-nuclear maxima might be a basis set-dependent effect, originally made by its discoverer [21], was not borne out by later studies [23,24].…”

“…The balance between core and valence correlation may be at least as important, in this respect, as virial compliance and proper description of the nuclear cusps, as might be suggested by a comparison of largest basis set results at the SCF and CISD (no quadruples) levels [22,23]. The configurations included in the OBS-GMCSC wavefunction (Table VI) were selected partly in an attempt to achieve this balance.…”

“…For the second line of work, electron density studies [19 -24] have shown that the finding of a non-nuclear maximum in it over a range of internuclear distances comprising the equilibrium distance R e is essentially independent of the level of theory employed for the calculation of the electronic wavefunction and have characterized the electron density's topological structure [21,23,24] and the changes it undergoes with internuclear distance, locating its "bifurcation points. "…”