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

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 7s6p5d3f2g1h and N 4s3p1d 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 (approximately 0.5 electron) along the transition. Our calculations are compared to reference coupled-cluster estimations.