Multi and Single-Reference Methods for the Analysis of Multi-State Peroxidation of Enolates
In spite of being spin-forbidden, some enzymes are capable of catalyzing the incorporation of O<sub>2</sub> (<sup>3</sup>Σ<sup>−</sup><sub>g</sub>) to<br>organic substrates without needing any cofactor. It has been established that the process followed by these<br>enzymes starts with the deprotonation of the substrate forming an enolate. In a second stage, the peroxidation<br>of the enolate formation occurs, a process in which the system changes its spin multiplicity from a triplet state<br>to a singlet state. In this article, we study the addition of O<sub>2</sub> to enolates using state-of-the-art multi-reference<br>and single-reference methods. Our results confirm that intersystem crossing is promoted by stabilization of<br>the singlet state along the reaction path. When multi-reference methods are used, large active spaces are<br>required, and in this situation, Semistochastic Heat-Bath Configuration Interaction (SHCI) emerges as a<br>powerful method to study these multi-configurational systems and is in good agreement with LCCSD(T)<br>when the system is well-represented by a single-configuration.<br><br>