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.