Spin–resolved analysis of electronegativity equalization and electron flow in molecules

Abstract

Individual spin contributions to electron flow in molecules are analyzed with concepts of density functional theory. The intra- and interspin components of the bond hardness tensor are rigorously defined and expressed in terms of the second-order response properties. Stability conditions with respect to charge transfer and spin polarization are formulated for closed-shell systems and the relative magnitudes of the bond and spin bond hardnesses are investigated. The energetics of electron flow is illustrated using the examples of the formaldehyde molecule and its radical cation. While the inclusion of electron correlation is found to have only a relatively minor effect on the computed bond hardness tensors, comparison of the data obtained at the unrestricted and restricted open-shell Hartree–Fock levels of theory uncovers the complete uselessness of the latter method in studies of spin–polarized electron flow. Test calculations show that several diatomic molecules suffer from spin–polarization instabilities at the UMP2/6-311++G** level of theory.