Development of Programs for Spin Projection, Energy Derivatives and Advanced Graphics in MO Calculations

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This project is being funded under the Computational Science and Engineering Initiative in the Theoretical and Computational Physical Chemistry program. Professor Schlegel's aim is to develop techniques for rapid calculation of reaction pathways for chemical reactions and the accurate theoretical prediction of energy barriers and bond- breaking energies. This work, and prior work by Professor Schlegel, are expected to be of great utility to experimental chemists. When unrestricted Hartree-Fock and Moeller-Plesset perturbation theory are used for reactions of open shell systems and for bond dissociations, contamination from unwanted spin states can sometimes be large enough to distort the energy surface severely. Annihilation of the largest spin contaminant dramatically improves calculated barrier heights and bond dissociation potentials. The present project will address the annihilation of all spin contaminants, calculation of gradients of spin-projected energies, spin annihilation applied to correlation corrections, and full CI and high order perturbation theory to test the spin projected methods. Chemical applications to demonstrate the utility of the spin projected method will include studies of radical additions to multiple bonds, radical abstraction reactions, bond dissociation potentials, and diradical intermediates in cycloaddition reactions. New algorithms will also be developed for gradients of spin projected energies, transition state optimization, reaction path following, basis set optimization using analytical gradients, and IR intensities using electric field-dependent functions.