Full Correlation in a Multiconfigurational Study of Bimetallic Clusters: Restricted Active Space Pair-Density Functional Theory Study of [2Fe-2S] Systems

Iron-sulfur clusters play a variety of important roles in protein chemistry, and understanding the energetics of their spin ladders is an important part of understanding these roles. Computational modeling can offer considerable insight into such problems; however, calculations performed thus far on systems with multiple transition metals have typically either been restricted to a single-configuration representation of the density, as in Kohn-Sham theory, or been limited to correlating excitations only within an active space, as in active-space self-consistent field methods. For greater reliability, a calculation should include full correlation, i.e., not only correlation internal to the active space but also external correlation, and it is desirable to combine this full electron correlation with a multiconfigura-tional representation of the wave function; but this has been impractical thus far. Here we present an affordable way to do that by using restricted-active-space pair-density functional theory. W...