H₂ binding to the active site of [NiFe] hydrogenase studied by multiconfigurational and coupled-cluster methods

[NiFe] hydrogenases catalyse the reversible conversion of molecular hydrogen to protons and electrons. This seemingly simple reaction has attracted much attention because of the prospective use of H₂ as a clean fuel. In this paper, we have studied how H₂ binds to the active site of this enzyme. Combined quantum mechanical and molecular mechanics (QM/MM) optimisation was performed to obtain the geometries, using both the TPSS and B3LYP density-functional theory (DFT) methods and considering both the singlet and triplet states of the Ni(ii) ion. To get more accurate energies and obtain a detailed account of the surroundings, we performed calculations with 819 atoms in the QM region. Moreover, coupled-cluster calculations with singles, doubles, and perturbatively treated triples (CCSD(T)) and cumulant-approximated second-order perturbation theory based on the density-matrix renormalisation group (DMRG-CASPT2) were carried out using three models to decide which DFT methods give the most accurate structures and energies. Our calculations show that H₂ binding to Ni in the singlet state is the most favourable by at least 47 kJ mol^-1. In addition, the TPSS functional gives more accurate energies than B3LYP for this system.