• Nonlocal electromagnetic response
• Intermolecular forces
• Dielectric theory on the nanoscale
• Polarizabilities
• Nonlinear optical properties
• Connections among spectroscopic observables
• Interaction effects on electrical properties
• Collision-induced spectroscopy

• Thermodynamic and stochastic theory of nonequilibrium systems
• Steady states: transitions and relative stability
• Electrochemical measurements on steady states
• Fluctuations in nonequilibrium systems
• Path integral techniques, Fokker-Planck equations

We are deriving new interconnections among spectroscopic observables, by use of nonlocal polarizability densities, response tensors that account for the distribution of polarizability within a molecule. Areas of application include molecular potential energy surfaces, intermolecular forces, infrared absorption, vibrational Raman scattering, vibrational circular dichroism, and intramolecular screening of intermolecular forces. Specific accomplishments by my group include:

• Proof of Feynman's 1939 "conjecture" about the origin of dispersion forces between atoms in S states, and proof of its generalization to molecules of arbitrary symmetry
• Derivation of new interrelations among electrostatic interactions, induction, and hyperpolarization
• Explanation of a set of counter-intuitive relations that connect electric field shielding tensors to the intensities of infrared absorption and vibrational Raman scattering bands
• Proof that permanent charge moments, linear response, and nonlinear response are all closely intertwined
• Proof of a relation between electronic hyperpolarizabilities and nuclear anharmonicities

In molecular quantum mechanics, my recent collaborators have included Prof. D. M. Bishop (Ottawa), Prof. P. W. Fowler (Exeter, UK), Prof. R. A. Harris (Berkeley), and Prof. A. J. Sadlej (Lund), as well as post-docs X. Li and Olga Jenkins, and Ph.D. students, J. E. Bohr, J. Juanos i Timoneda, Y. Q. Liang, R. Nimalakirthi, P. H. Liu, Ed Tisko, Sandjaja Tjahajadiputra, Chetan Ahuja, and Mark Champagne. Currently, X. Li, C. Ahuja, and I are collaborating with Prof. James F. Harrison (Michigan State University) on the ab initio calculation of nonlocal charge density susceptibilities.

We have also been developing a thermodynamic and stochastic theory of nonequilibrium systems, in collaboration with Prof. John Ross (Stanford) and Prof. Paul M. Hunt (Michigan State University). This theory applies to reacting chemical systems with fluxes of matter or energy across the boundaries. We have derived an "excess work" that acts analogously to the Gibbs free energy in equilibrium thermodynamics, in several regards:

• The reversible excess work is a state function
• The excess work vanishes at steady states, and its sign provides a criterion for the stability of steady states
• The excess work decreases monotonically as a system evolves deterministically, and it increases monotonically as the system moves away from steady state on a fluctuational trajectory
• The excess work differs from the total work by an amount necessary to maintain the constraints on the system
• The excess work determines the probability of fluctuations away from the steady state

Prof. John Ross, Prof. Paul Hunt, and I have worked jointly with post-doctoral associates Q. Zheng, X. Chu, and B. Peng. In addition, many recent members of the Ross group have been valuable collaborators in this area.

Current Research Topics

• Dielectric functions within molecules
• with Olga Jenkins
• Charge density susceptibilities calculated ab initio
• with Jim Harrison, Xiaoping Li, and Chetan Ahuja
• Electrochemical potentials in nonequilibrium systems: Deviations from the Nernst equation due to dissipation
• with John Ross, Paul Hunt, Alan Hjelmfelt, and Masa Yamato
• Nonlocal current density susceptibilities
• with Ed Tisko
• Collision induced absorption by nitrogen/methane mixtures
• with Xiaoping Li and Mark Champagne

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