title: Magnetic gaps in organic tri-radicals: From a simple model to accurate estimates.
abstract: The calculation of the energy gap between the magnetic states of organic poly-radicals still represents a challenging playground for quantum chemistry, and high-level techniques are required to obtain accurate estimates. On these grounds, the aim of the present study is twofold. From the one side, it shows that, thanks to recent algorithmic and technical improvements, we are able to compute reliable quantum mechanical results for the systems of current fundamental and technological interest. From the other side, proper parameterization of a simple Hubbard Hamiltonian allows for a sound rationalization of magnetic gaps in terms of basic physical effects, unraveling the role played by electron delocalization, Coulomb repulsion, and effective exchange in tuning the magnetic character of the ground state. As case studies, we have chosen three prototypical organic tri-radicals, namely, 1,3,5-trimethylenebenzene, 1,3,5-tridehydrobenzene, and 1,2,3-tridehydrobenzene, which differ either for geometric or electronic structure. After discussing the differences among the three species and their consequences on the magnetic properties in terms of the simple model mentioned above, accurate and reliable values for the energy gap between the lowest quartet and doublet states are computed by means of the so-called difference dedicated configuration interaction (DDCI) technique, and the final results are discussed and compared to both available experimental and computational estimates.
authors.affiliation: Scuola Normale Superiore di Pisa, Piazza dei Cavalieri, I-56126 Pisa, Italy.
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Barone
authors.affiliation: Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Giuseppe Moruzzi 13, I-56124 Pisa, Italy.
authors.forename: Ivo
authors.identifier: 0000000199411800
authors.initials: I
authors.lastname: Cacelli
authors.affiliation: Istituto di Chimica dei Composti OrganoMetallici (ICCOM-CNR), Area della Ricerca, Via Giuseppe Moruzzi 1, I-56124 Pisa, Italy.
authors.forename: Alessandro
authors.identifier: 000000033066605X
authors.initials: A
authors.lastname: Ferretti
authors.affiliation: Istituto di Chimica dei Composti OrganoMetallici (ICCOM-CNR), Area della Ricerca, Via Giuseppe Moruzzi 1, I-56124 Pisa, Italy.
authors.forename: Giacomo
authors.initials: G
authors.lastname: Prampolini