Performance of CASPT2 and DFT for Relative Spin-State Energetics of Heme Models

Abstract

The accuracy of the relative spin-state energetics of three small Fe(II) or Fe(III) heme models from multiconfigurational perturbation theory (CASPT2) and density functional theory with selected functionals (including the recently developed M06 and M06-L functionals) was assessed by comparing with recently available coupled cluster results. While the CASPT2 calculations of spin-state energetics were found to be very accurate for the studied Fe(III) complexes (including FeP(SH), a model of the active site of cytochrome P450 in its resting state), there is a strong indication of a systematic error (around 5 kcal/mol) in favor of the high-spin state for the studied Fe(II) complexes (including FeP(Im), a model of the active site of myoglobin). A larger overstabilization of the high-spin states was observed for the M06 and M06-L functionals, up to 22 and 11 kcal/mol, respectively. None of the tested density functionals consistently provides a better accuracy than CASPT2 for all model complexes.

Towards quantifying the role of exact exchange in predictions of transition metal complex properties

“…49 Wavefunction approaches are not without challenges and in some cases still produce sizeable, 5 kcal/mol energetic errors in spin-state ordering. 48 However, they are typically in very good agreement with experimental spin crossover properties and are a suitable reference for benchmarking of exchange-correlation functionals for higher throughput studies. Despite advances in wavefunction theory, approximate density functionals are still preferred by most computational researchers due to ease of use and lower scaling that makes geometry optimization and high-throughput calculations feasible.…”

“…For a balanced treatment of both static correlation and dynamic correlation also in the absence of self-interaction error, multireference wavefunction techniques have been used [43][44][45][46][47][48][49][50] to study spin crossover complexes up to around 45 atoms in size. 48 The predominant wavefunction method employed in the study of spin-crossover complexes is CASPT2, and, while it scales more expensively than density functional theory approaches, recent improvements in scaling 44 have made larger systems tractable. Other studies have applied the even more expensively scaling coupled-cluster to smaller spin crossover complexes.…”

Computational approach to the study of thermal spin crossover phenomena

“…[45][46][47][48][49][50][51][52][53][54][55][56][57] Multiconfigurational wave function based methods, on the other hand, have proved to be capable of giving accurate values for the LS-HS energy difference but they are computationally much more expensive, particularly in the calculation of optimized geometries and vibrational frequencies. 26,28,29,51,52,[58][59][60][61][62] The aim of this work is to determine through calculations the key parameters of the thermal SCO process, i.e., the zeropoint corrected energy difference between the LS and HS states, ?H HL , the entropy change associated to the spin transition, ?S HL , and an estimation of the transition temperature, T 1/2 , for a set of Fe(II) compounds with ligands of different nature. In order to do that, we have combined DFT calculations on the geometries and vibrational frequencies for the LS and HS states, with multiconfigurational wave function calculations that allow us to compute accurate electronic energy differences.…”

Protein effects in non-heme iron enzyme catalysis: insights from multiscale models

“…[45] This is partly due to the need for a large basis sets. Effects of 5 kcal/mol on the relative spin-state energies of a heme iron system have been observed when going from a triple to a quadruple-zeta basis set on iron, [46] while DFT results are expected to converge at the triple-zeta level. The multiconfigurational methods have yet to find widespread use for non-heme iron enzymes, but can be expected to increase in frequency.…”