Analytic Gradients for the Spin-Flip ORMAS-CI Method: Optimizing Minima, Saddle Points, and Conical Intersections.

Analytic nuclear gradients are derived and implemented for the recently introduced SF-ORMAS-CI (spin-flip occupation restricted multiple active space CI) method. Like most SF methods, SF-ORMAS-CI successfully describes bond breaking, diradical systems, transition states, and low-lying excited states, without suffering from spin contamination. The availability of analytic gradients now enables the efficient optimization of equilibrium structures in both ground and excited electronic states, as well as the computation of seminumerical Hessians. Therefore, it is now possible to determine minima, transition states, and conical intersections using the SF-ORMAS-CI method without the need for numerical differentiation. In the present study the SF-ORMAS method and gradient are applied to optimize structures for several organic molecules, such as ethylene, azomethane, and trimethylmethylene. In most cases, structures optimized with SF-ORMAS are almost identical to those obtained using other multireference methods, despite the lack of dynamic correlation in SF-ORMAS.