Performance of Molecular Orbital Methods and Density Functional Theory in the Computation of Geometries and Energies of Metal Aqua Ions

Abstract

Geometries and energies of various metal aqua ions have been computed with Hartree-Fock (HF), single-reference second-order perturbation theory according to Møller and Plesset (MP2), complete active space self-consistent field (CAS-SCF) methods, multi-configurational quasi-degenerate second-order perturbation theory (MCQDPT2), and density functional theory (DFT), whereby for the latter the most widely used functionals BLYP, B3LYP, LDA ("local density approximation"), and another functional (SOP) exhibiting Slater exchange, have been investigated. The geometries as well as the energies are sensitive to the computational method, results of which can lead to different conclusions. The geometries of certain reactants, products, intermediates, or transition states cannot be obtained with all of the above-mentioned computational methods: in some cases, the structures are very poor, exhibiting large errors in the bond parameters, and in others, the targeted species cannot be calculated at all. The different computational techniques do not always predict the same coordination number of a given aqua ion or the same preferred water exchange mechanism. Inaccuracies arising from inadequate basis sets are also discussed.

Computational insights into the O2-evolving complex of photosystem II

“…These observation have been recently addressed by calculations of transition state energy barriers for water exchange in structural models of the OEC in the S 1 and S 2 states while progressively detaching substrate water molecules from Ca 2+ and the dangling Mn(4) (Sproviero et al 2007f). The resulting structural rearrangements provided insight on the water exchange mechanisms and the relative binding strengths, considering that elongation of the metal-oxygen bond is likely the primary step in water exchange and rate-determining (Lundberg et al 2003;Rotzinger 1997;Rotzinger 2005). These calculations complemented earlier studies of water exchange in transition metal complexes (Cady et al 2006;Helm & Merbach 1999;Houston et al 2006;Rotzinger 1997;Rotzinger 2005;Tagore et al 2006;Tagore et al 2007), including theoretical studies of manganese complexes, based on Hartree-Fock and complete active-space self-consistent field theories (Lundberg et al 2003;Rotzinger 1997;Rotzinger 2005;Tsutsui et al 1999) as well as DFT studies of water exchange in other transition metal complexes (Deeth & Elding 1996;Hartmann et al 1999;Hartmann et al 1997;Lundberg et al 2003;Vallet et al 2001).…”

QM/MM computational studies of substrate water binding to the oxygen-evolving centre of photosystem II

“…Mn (4) Mn (3) Mn (2) bond is the primary step in water exchange and presumably rate determining in this case (Rotzinger 1997;Lundberg et al 2003;Rotzinger 2005). Figure 3 shows the resulting energy profiles for the MEPs, while progressively detaching substrate water molecules from Ca 2C and the dangling manganese for the OEC in the S 1 and S 2 states Mn 4 (IV,IV,III,III ) and Mn 4 ( IV,IV,IV,III ), respectively.…”

A Gas-phase study of the preferential solvation of Mn2+ in mixed water/methanol clusters

“…However, a recent review by Rotzinger [27] on the performance of molecular orbital methods and density functional theory in the computation of geometries and energies of metal aqua ions has shown that DFT has several limitations. In particular, the metalligand and hydrogen bond strengths are not balanced correctly, especially in complexes exhibiting a high (Ն2) charge, and current popular functionals appear to favor lower coordination number.…”