METHODS FOR FINDING UNRESTRICTED HARTREE-FOCK SOLUTIONS AND MULTIPLE SOLUTIONS

The unrestricted Hartree-Fock (UHF) wave function is fundamental for the description of open-shell systems. However, finding solutions to the UHF equations is often not trivial, particularly in systems with an even number of electrons. In addition, the existence of multiple solutions is more widespread than assumed hitherto. On the basis of the theory of triplet instability, we describe methods that automatically find the existing UHF solutions. We also analyze the nature of multiple solutions. The method is illustrated with a number of examples: ozone, the nitrite anion, stretched LiH, dilithium, dioxygen difluoride, and nitrogen oxide. © 1990 American Chemical Society.