Toward Highly Accurate Spin State Energetics in First-Row Transition Metal Complexes: A Combined CASPT2/CC Approach

• Published in: Journal of chemical theory and computation 2018-05, Vol.14 (5), p.2446-2455
• Main Authors: Phung, Quan Manh, Feldt, Milica, Harvey, Jeremy N, Pierloot, Kristine
• Format: Article
• Language: English
• Subjects: Bias; Catalysis; Coordination compounds; Correlation; Organic chemistry; Perturbation theory; Transition metal compounds
• ISSN: 1549-9618; 1549-9626
• DOI: 10.1021/acs.jctc.8b00057

In previous work on the performance of multiconfigurational second-order perturbation theory (CASPT2) in describing spin state energetics in first-row transition metal systems [Pierloot et al. J. Chem. Theory Comput. 2017, 13, 537−553 ], we showed that standard CASPT2 works well for valence correlation but does not describe the metal semicore (3s3p) correlation effects accurately. This failure is partially responsible for the well-known bias toward high-spin states of CASPT2. In this paper, we expand our previous work and show that this bias could be partly removed with a combined CASPT2/CC approach: using high-quality CASPT2 with extensive correlation-consistent basis sets for valence correlation and low-cost CCSD­(T) calculations with minimal basis sets for the metal semicore (3s3p) correlation effects. We demonstrate that this approach is efficient by studying the spin state energetics of a series of iron complexes modeling important intermediates in oxidative catalytic processes in chemistry and biochemistry. On the basis of a comparison with bare CCSD­(T) results from this and previous work, the average error of the CASPT2/CC approach is estimated at around 2 kcal mol–1 in favor of high spin states.