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 insights 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, that is, not only correlation internal to the active
space but also external correlation, and it is desirable to combine
this full-electron correlation with a multiconfigurational representation
of the wave function, but this has been impractical thus far. Here,
we present an affordable way to do this by using restricted active-space
pair-density functional theory. We show that with this method, it
is possible to compute the entire spin ladder for systems containing
two Fe centers bridged by two S atoms. On the other hand, with second-order
perturbation theory, only the high-spin states can be computed. A
key result is that, in agreement with some experiments, we find a
high-spin ground state for a relaxed reduced [Fe2S2(SCH3)4]3– cluster,
which is a novel result in computational studies.