title: On the zeroth-order hamiltonian for CASPT2 calculations of spin crossover compounds.
abstract: Complete active space self-consistent field theory (CASSCF) calculations and subsequent second-order perturbation theory treatment (CASPT2) are discussed in the evaluation of the spin-states energy difference (ΔH(elec)) of a series of seven spin crossover (SCO) compounds. The reference values have been extracted from a combination of experimental measurements and DFT + U calculations, as discussed in a recent article (Vela et al., Phys Chem Chem Phys 2015, 17, 16306). It is definitely proven that the critical IPEA parameter used in CASPT2 calculations of ΔH(elec), a key parameter in the design of SCO compounds, should be modified with respect to its default value of 0.25 a.u. and increased up to 0.50 a.u. The satisfactory agreement observed previously in the literature might result from an error cancellation originated in the default IPEA, which overestimates the stability of the HS state, and the erroneous atomic orbital basis set contraction of carbon atoms, which stabilizes the LS states.
authors.affiliation: Laboratoire de Chimie Quantique, UMR 7177, Université de Strasbourg, 4 rue Blaise Pascal, F-67000 Strasbourg, France.|Departament de Química Física and IQTCUB, Universitat de Barcelona, Av. Diagonal 645, 08028, Barcelona, Spain.
authors.forename: Sergi
authors.initials: S
authors.lastname: Vela
authors.affiliation: Departament de Química Física and IQTCUB, Universitat de Barcelona, Av. Diagonal 645, 08028, Barcelona, Spain.
authors.forename: Maria
authors.initials: M
authors.lastname: Fumanal
authors.affiliation: Departament de Química Física and IQTCUB, Universitat de Barcelona, Av. Diagonal 645, 08028, Barcelona, Spain.
authors.forename: Jordi
authors.initials: J
authors.lastname: Ribas-Ariño
authors.affiliation: Laboratoire de Chimie Quantique, UMR 7177, Université de Strasbourg, 4 rue Blaise Pascal, F-67000 Strasbourg, France.
authors.forename: Vincent
authors.initials: V
authors.lastname: Robert
keywords: computational chemistry; molecular magnetism; spin crossover