Response properties and stability conditions in density matrix functional theory

Expressions for the second-order energy variations in the density matrix fu nctional theory (DMFT) are derived, resulting in a formalism for time-indep endent response properties (including absolute electronegativity and hardne ss) and stability conditions. A quadratically convergent scheme for a direc t determination of natural spinorbitals and their occupancy numbers is deve loped and tested with the Goedecker-Umrigar and the exact two-electron func tionals. The derivatives of the electronic energy with respect to the numbe r of electrons are found to be very sensitive to the DMFT description of th e exchange-correlation energy, providing a sensitive measure of accuracy th at can be readily employed in testing and development of approximate functi onals. (C) 2001 American Institute of Physics.