DENSITY-FUNCTIONAL THEORY OF BORN COUPLINGS - CONSEQUENCES FOR ELECTRON FLOW IN JAHN-TELLER MOLECULES AND SUPERCONDUCTORS

The dynamics of Jahn-Teller systems has recently been discussed in ter ms of generalized electronic charge and current densities in nuclear-c oordinate space. The introduction of the electronic phase as a functio n of both electronic and nuclear coordinates, in addition to the elect ronic density, was a crucial component of this formulation Here, a den sity-based treatment of Born couplings is derived from first-principle s quantum mechanics beyond the Born-Oppenheimer approximation. Because of the degenerate electronic configuration of a Jahn-Teller molecule, there are an infinite number of ways in which the charge distribution can be oriented for the same energy, leading to a vanishing bond hard ness for the molecule in the symmetric nuclear configuration. Further, the moving nuclear framework serves as the perturbation necessary to define the orientation of the charge density, leading to unhindered ro tation of the charge cloud. This leads to the dynamical Jahn-Teller pr oblem, namely, the coupling of electronic and nuclear motions through the Born coupling terms. Applications to superconductivity theory are discussed. (C) 1995 John Wiley & Sons, Inc.