Bond functions and the topological properties of the bonds

Abstract

The effect of bond functions on the topological properties of the bonds is studied by performing Hartree-Fock calculations for 02, Ch_, F,., CN-, N,. and LiCI molecules. The results show that bond functions increse the degree of covalency of the bonds and the electronic density at the (3, -1) bond critical point. This effect is quite noticeable for intermediate interactions molecules where a V2pcrit <~ 0 is obtained instead of a positive value. The optimal position of the bond functions seems to be at or near the (3, -1) bond critical point.KEY WORDS: Bond functions; topology; Laplacian; bond properties. ~TRODUCTIONBader's topological theory [1,2] has become a very useful tool for the interpretation of results obtained from molecular quantum calculations. In this theory, the chemical bonds are characterized without using any orbital formalism and the analysis of atomic bonding properties is performed in terms of the total molecular electronic density (p (r)) and the Laplacian of p (r), at the bond critical points (Pcrit, V2pcrit) [1,2]. In the theory Atoms in a Molecule [2], the interatomic interactions are classified as (a) shared interaction, when the Pent value is high, V2pcrit < 0, [)kl[/)k 3 > 1 and acrit/Pcri t < l; (b) closed-sheU interaction, when the P~t value is low, gr2pcri t > 0, Ix~l/~,3 < 1 and Gent~Pent > 1. The interactions that do not fulfill the shared or closed-shell interaction models are called intermediate interactions. It has been stressed [3] that although the Pcrit value is quite insensitive to the choice of the atomic basis functions, the laplacian of p (r) at the bond critical point shows larger variations with the basis set size. On the other hand, it is usually assumed that the Hartree-Fock elec- 333 tronic densities, calculated using only nucleus-centered atomic functions, are good enough in order to classify the interatomic interactions, at least for those molecules where it is expected that the electronic bond correlation is not so high to have a strong impact on the topological properties of the bonds, such as the case of the CO molecules where at SCF Hartree-Fock level vZPcrit > 0, but in the CASSCF approach [4], V2p:nt becomes negative (V2p:d~ = -2.101 au).A different approach to the only-nucleus-centered ab initio molecular calculation, that located additional polarization functions at the bond midpoint (bond function or BF), has been extensively studied and recommended by several authors [5][6][7][8][9][10] for the calculation of molecular dissociation energy (De) and spectroscopic diatomic constants. The LCBA approximation (linear combination of bond and atomic functions) usually gives closer values to the experimental D e and for the other spectroscopic constants such as ~0 e, o~ e Xe, Be and ~e [6,10] than calculations performed with only nucleus-centered atomic functions (LCAO approximation). However, the usefulness of the BF has been clouded by the basis set superposition error (BSSE) problem I11]. Since the topology of p (r) is related to the interatomic interactio...