title: Communication: Novel quantum states of electron spins in polycarbenes from ab initio density matrix renormalization group calculations.
abstract: An investigation into spin structures of poly(m-phenylenecarbene), a prototype of magnetic organic molecules, is presented using the ab initio density matrix renormalization group method. It is revealed by achieving large-scale multireference calculations that the energy differences between high-spin and low-spin states (spin-gaps) of polycarbenes decrease with increasing the number of carbene sites. This size-dependency of the spin-gaps strikingly contradicts the predictions with single-reference methods including density functional theory. The wave function analysis shows that the low-spin states are beyond the classical spin picture, namely, much of multireference character, and thus are manifested as strongly correlated quantum states. The size dependence of the spin-gaps involves an odd-even oscillation, which cannot be explained by the integer-spin Heisenberg model with a single magnetic-coupling constant.
authors.affiliation: The Graduate University for Advanced Studies, Myodaiji, Okazaki, Aichi 444-8585, Japan. mizukami@ims.ac.jp
authors.forename: Wataru
authors.initials: W
authors.lastname: Mizukami
authors.forename: Yuki
authors.initials: Y
authors.lastname: Kurashige
authors.forename: Takeshi
authors.initials: T
authors.lastname: Yanai