Constructing Molecular π-Orbital Active Spaces for Multireference Calculations of Conjugated Systems

Molecules with conjugated pi systems often feature strong electron correlation and therefore require multi-reference methods for a reliable computational description. A key prerequisite for the successful application of such methods is the choice of a suitable active space. Herein the automated pi-orbital space (PiOS) method for selecting active spaces for multireference calculations of conjugated pi systems is presented. This approach allows the construction of small but effective active spaces based on Huckel theory. To demonstrate its performance, pi -> pi* excitations for benzene, octatetraene, and free-base porphine are computed. In addition, this technique can be combined with the automated atomic valence active space method to compute excitations in complex systems with multiple conjugated fragments. This combined approach was used to generate two-dimensional potential energy surfaces for multiple electronic states associated with photoinduced electron-coupled double proton transfer in the blue-light-using flavin photoreceptor protein. These types of methods for the automated selection of active space orbitals are important for ensuring consistency and reproducibility of multireference approaches for a wide range of chemical and biological systems.