Molecular-Orbital Theory of Diamagnetism. I. An Approximate LCAO Scheme

Abstract

An independent‐electron molecular‐orbital theory is developed for the diamagnetic behavior of electrons in the presence of an applied magnetic field. The molecular orbitals are written as linear combinations of gauge‐invariant atomic orbitals, the dependence of the (complex) coefficients on the magnetic field being studied by perturbation theory. By making a systematic set of approximations involving the neglect of some interatomic terms, a general expression is derived for the diamagneticsusceptibilitytensor as a sum of atomic contributions. Each atomic contribution is made up of two parts, the first being a diamagnetic (Langevin‐type) term and the second being a paramagnetic contribution involving the details of the electronic excited states. At a lower level of approximation, this second term can be expressed in terms of atomic charge densities and bond orders together with a mean electronic excitation energy. It is pointed out that the theory provides a convenient basis for detailed interpretation of the Pascal constants used for empirical calculations of diamagneticsusceptibilities of large molecules. In addition, a similar treatment of the theory of nuclear magnetic screening leads to an expression for the shielding constant involving a set of localized contributions closely related to corresponding contributions for the susceptibility.