title: Communication: an efficient analytic gradient theory for approximate spin projection methods.
abstract: Spin polarized and broken symmetry density functional theory are popular approaches for treating the electronic structure of open shell systems. However, spin contamination can significantly affect the quality of predicted geometries and properties. One scheme for addressing this concern in studies involving broken-symmetry states is the approximate projection method developed by Yamaguchi and co-workers. Critical to the exploration of potential energy surfaces and the study of properties using this method will be an efficient analytic gradient theory. This communication introduces such a theory formulated, for the first time, within the framework of general post-self consistent field (SCF) derivative theory. Importantly, the approach taken here avoids the need to explicitly solve for molecular orbital derivatives of each nuclear displacement perturbation, as has been used in a recent implementation. Instead, the well-known z-vector scheme is employed and only one SCF response equation is required.
authors.affiliation: Gaussian, Inc., 340 Quinnipiac Street, Building 40, Wallingford, Connecticut 06492, USA. hrant@gaussian.com
authors.forename: Hrant P
authors.initials: HP
authors.lastname: Hratchian