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We study the origin of the cooperative nature of spin crossover (SC) between low-spin and high-spin (HS) states from the viewpoint of elastic interactions among molecules. As the size of each molecule changes depending on its spin state, the elastic interaction among the lattice distortions provides the cooperative interaction of the spin states. We develop a simple model of SC with intra and intermolecular potentials which accounts for the elastic interaction including the effect of the inhomogeneity of the spin states and apply constant temperature molecular dynamics based on the Nosé-Hoover formalism. We demonstrate that, with increase of the strength of the intermolecular interactions, the temperature dependence of the HS component changes from a gradual crossover to a first-order transition.

Pressure-induced phase transitions of spin-crossover materials are studied in a microscopic model taking into account the elastic interaction among distortions of lattice due to the difference of the molecular sizes between the high-spin state and the low-spin state. We perform Monte Carlo simulations in the constant pressure ensemble and reproduce several important properties of the pressure effect in a unified way with a microscopic mechanism for the first time. The simulation newly reveals how the temperature dependence of the ordering process changes with the pressure.

Direct exchange interactions between divalent chromium ions in naked dimers, tetramers, and clusters were investigated theoretically. The orbital energy gaps, occupation numbers, and bond orders for the naked Cr(II)-Cr(II) dimer (1) were calculated by ab initio UHF MO and density functional (DFT) methods. All these calculations indicated that the orbital energy gaps are very small for 1, and therefore the occupation numbers of the bonding σ, π, and δ molecular orbitals and formal quadruple bond orders were largely reduced because of strong electron correlation effects. The UHF and DFT MO calculations were also performed for the linear naked Cr(II) tetramer to elucidate possible electronic structures of the d-d conjugated systems. The energy differences among singlet, triplet, and quintet states for 1 were calculated from the total energies obtained by the CASCI and CASSCF methods based on the UHF and DFT natural orbitals (UNO). The reliability of the Heisenberg model Hamiltonian was elucidated from variations of these calculated effective exchange integrals with the interatomic distance. The calculated J ab values are compared with the experimental results for several binuclear Cr(II) complexes with different interatomic distances. The Heisenberg spin Hamiltonian was used to discuss the spin alignments in large d-d conjugated Cr(II) clusters and a possibility of the macroscopic quantum spin tunneling in mesoscopic magnetic systems.

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