Analytical energy gradient of high-spin multiplet state calculated by the SAC-CI method

A method of calculating analytical energy gradient of high-spin multiplet state by the SAC-CI (symmetry-adapted-cluster configuration-interaction) method is developed and implemented. This method is expected to be a powerful tool in studying the dynamics and properties of molecules having high spin-multiplicities. Good performance of this method is shown for the quartet states of BH+ and C2+ and for the quintet states of C2. The SAC-CI general-R method is also extended to the high-spin states, and proved to be useful especially for calculating accurate adiabatic excitation energies of the systems having quasi-degenerate orbital structure.