Physics

The cohesion in the material (BDTA)2[Co(mnt)2] (1) is analyzed in the light of experimental data and ab initio calculations (BDTA = 1,3,2-benzodithiazolyl, and mnt2 = maleonitrildithiolate). It is demonstrated that the duality of the thiazyl radical BDTA as cation and ligand stems from the competition between the Co(II) ion–sulfur-rich part of the ligand interactions (i.e. s-type interactions), and the equatorial ligand mnt2–benzyl moiety contacts (i.e. p–p interactions). The molecular changes observed between the low-temperature and high-temperature regimes are interpreted on the basis of multireference CASSCF wavefunction and subsequent CASPT2 calculations using an original Lewis-like picture. The Jahn–Teller-like distortion is accompanied by an unusual electronic redistribution affecting the s-type and p–p interactions, leading to short (2.65 A ˚ ) and long (3.48 A ˚ ) Co–BDTA distances in good agreement with X-ray data. Due to its intrinsic architecture and radical character, the BDTA ligand is simultaneously involved in weak metal–ligand and ligand–ligand bonds. Such lability in bond formations is of prime importance in bistable materials preparation. The role traditionally attached to the metal center in transition metal complexes is reconsidered in systems such as 1, where significant charge transfers are quantified between the low and high temperature phases.

The design of bistable magnetic systems should enable the storage of information by manipulation of the spin degrees of freedom. However, such a strategy relies on the preparation of target objects, whose environment must be controlled to favor a hysteretic behavior. Here, we report the successful modeling of a highly cooperative two-step spincrossover iron(II) compound, [Fe(bapbpy)(NCS)2]. The magnetic susceptibility measurements and low- and hightemperature hysteretic cycles reflect the presence of an intermediate phase, which controls the memory-storage capacity of this material. It is shown that the hysteresis loop widths can be traced theoretically by evaluating the electrostatic contributions between the transiting units. Despite the apparent reduction of intermolecular interactions upon cooling, it is suggested that the enhanced fluctuations of the Madelung field are responsible for the observed hysteresis width changes. This counterintuitive scenario makes the preparation of information storage devices an even more challenging task, where theoretical inspections are very insightful.

The importance of basis sets and active spaces in the determination of the potential energy curves and relevant energy differences in the Oh-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 7s6p5d3f 2g1h 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 (∼0.5 electron) along the transition. Our calculations are compared to reference coupled-cluster estimations.

We report a comprehensive analysis of the hysteresis behavior in a series of well-characterized spin-crossover Fe(II) materials. On the basis of the available X-ray data and multireference CASSCF (complete active space self-consistent field) calculations, we show that the growth of the hysteresis loop is controlled by electrostatic contributions. These environment effects turn out to be deeply modified as the crystal structure changes along the spin transition. Our theoretical inspection demonstrates the synergy between weak bonds and electrostatic interactions in the growth of hysteresis behavior. Quantitatively, it is suggested that the electrostatic contributions significantly enhance the cooperativity factor while weak bonds are determinant in the structuration of the 3D networks. Our picture does not rely on any parametrization but uses the microscopic information to derive an expression for the cooperativity parameter. The calculated values are in very good agreement with the experimental observations. Such inspection
can thus be carried out to anticipate the hysteresis behavior of this intriguing class of materials.

A simple algebraic approach to the study of multipartite entanglement for pure states is introduced together with a class of suitable functionals able to detect entanglement. On this basis, some known results are reproduced. Indeed, by investigating the properties of the introduced functionals, it is shown that a subset of such class is strictly connected to the purity. Moreover, a direct and basic solution to the problem of the simultaneous maximization of three appropriate functionals for three-qubit states is provided, confirming that the simultaneous maximization of the entanglement for all possible bipartitions is compatible only with the structure of GHZ-states.