Subsystem Functionals in Density Functional Theory

Ann E. Mattsson, Sandia National Laboratories, Albuquerque, NM

Rickard Armiento, Royal Institute of Technology, Stockholm, Sweden

A viable way of extending the successful use of density functional theory into studies of even more complex systems than are addressed today has been suggested by Kohn and Mattsson [W. Kohn and A. E. Mattsson, Phys. Rev. Lett. 81, 3487 (1998); A. E. Mattsson and W.Kohn, J. Chem. Phys. 115, 3441 (2001)]. The scheme consists of dividing a system into subsystems and applying different approximations for the unknown (but general) exchange-correlation functional to the different subsystems. We discuss the basic requirement on approximate functionals used in this scheme; they must all adhere to a single explicit choice of the exchange-correlation energy per particle. From a numerical study of a model system with a cosine effective potential, the Mathieu gas, and one of its limiting cases, the harmonic oscillator model, we also show that the conventional definition of the exchange energy per particle must be modeled by a non-analytical function. We discuss the implications of our findings on the future of functional development.