Characterization of the weak SS bonds in the OSSSO and O2SSSO2 molecules.

The weak S1-S3 bonds in the OSSSO trans-disulfoxide and the corresponding sulfone, O(2)SSSO(2), are readdressed at the B3LYP/6-31+G(d) level using both the atoms-in-molecules (AIM) and the electron localization function (ELF) approaches. The S1-S3 bonds are clearly characterized as fractional (i.e., with a bond number or bond order which is less than unity) or protocovalent and are very similar in nature to the weak N-N bond in O(2)NNO(2). These results are in accord with what is obtained by inspection of valence bond structures of the increased-valence type.

Local density functional theory of atoms and molecules.

A LOCAL DENSITY FUNCTIONAL THEORY OF THE GROUND ELECTRONIC STATES OF ATOMS AND MOLECULES IS GENERATED FROM THREE ASSUMPTIONS: (i) The energy functional is local. (ii) The chemical potential of a neutral atom is zero. (iii) The energy of a neutral atom of atomic number Z is -0.6127 Z(7/3). The energy functional is shown to have the form [Formula: see text] where A(0)=6.4563 and B(0)=1.0058. The first term represents the electronic kinetic energy, the second term represents the electron-electron repulsion energy for N electrons, and the third term is the nucleus-electron attraction energy. The energy E and the electron density rho are obtained and discussed in detail for atoms; their general properties are described for molecules. For any system the density becomes zero continuously at a finite distance from nuclei, and contours of the density are contours of the bare-nuclear potential v. For an atomic species of fractional charge q = 1 - (N/Z), an energy formula is obtained, [Formula: see text] which fits Hartree-Fock energies of 625 atoms and ions with root-mean-square error of 0.0270. A more general local density functional involving a coefficient B(N) = B(0)N(2/3) + B(1) is briefly considered.

An atomic kinetic energy functional with full Weizsacker correction.

A functional is proposed for representing the electronic kinetic energy of the ground state of an N-electron atom or ion in terms of its electron density, [Formula: see text] Here T(w) is the Weizsacker quantity ((1/8))integral(nablarho.nablarho/rho)dtau and T(0) is the Thomas-Fermi quantity C(F) integral rho(5 / 3)dtau. From Hartree-Fock data on 55 neutral atoms, C = 1.412 +/- 0.033; for 1200 atoms and ions, C = 1.332 +/- 0.053. The proposed functional gives the derivative deltaT/deltarho its most important correct properties. The term T(w) is shown to give the kinetic energy of the K shell, whereas the term (C/N((1/3)))T(0) gives an incorrect statistical estimate of that energy. An alternative correction -(C/N((1/3)))T gives even better results.