Energetics of [Fe(NCH)6]2+ via CASPT2 calculations: A spin-crossover perspective

✍ Scribed by Mikaël Kepenekian; Vincent Robert; Boris Le Guennic; Coen De Graaf

• Publisher: John Wiley and Sons
• Year: 2009
• Tongue: English
• Weight: 224 KB
• Volume: 30
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/jcc.21236

✦ Synopsis

Abstract

The importance of basis sets and active spaces in the determination of the potential energy curves and relevant energy differences in the O~h~‐symmetry model system [Fe(NCH)~6~]^2+^ is analyzed using the Complete Active Space Self‐Consistent Field (CASSCF) method and subsequent second‐order perturbative treatment (CASPT2). By comparison of a series of atomic basis sets contraction, it is concluded that a balanced description of the Fe 7__s__6__p__5__d__3__f__2__g__1__h__ and N 4__s__3__p__1__d__ partners is needed to reach convergence upon the potential energy surface descriptions. Since the spin‐crossover phenomenon involves the simultaneous change in the spin nature and expansion of the coordination sphere of the metal ion (i.e., lengthening of the Fe‐N distances), the standard 10 electrons/12 orbitals complete active space is confronted to a chemically intuitive 18 electrons/15 orbitals picture. The role of a second d‐shell is finally examined using the extended RAS strategy. Using a valence‐bond type analysis, it is shown that the so‐called d^′^ orbitals allow for a significant charge redistribution (∼0.5 electron) along the transition. Our calculations are compared to reference coupled‐cluster estimations. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009