Theoretical Calculations of Structure and Exchange Coupling of a Room-Temperature Molecular Magnet

We report here quantum chemical calculations advancing structural hypotheses and corresponding methodological concerns devoted to the elucidation of magneto-structural features of the V[TCNE](2) molecular magnet (TCNE = tetracyanoethylenc) V[TCNE](x)nu(CH2Cl2) is the first room-temperature molecular magnet discovered and the only one with an active organic component. Despite previous detailed magnetic, spectroscopic and conductivity studies, the mechanism for the strong exchange coupling in the family M[TCNE](x)y(solvent) has remained an open question, the difficulty being related to the absence of structural data for these amorphous compounds Starting from a structural model that we propose, we report the results of Density Functional Theory (DFT) calculations providing the optimized geometry of a periodic lattice The DFT methods retrieve the long range magnetic ordering of the system, but overestimate the absolute values of exchange constants. We analyze the DFT results assessing the intrinsic limitations in the systematic account of the Broken Symmetry regime and long range effects. In this respect, Gaussian-type and Plane Wave calculations were corroborated, finding comparable results We suggest and preliminarily checked improved treatments realized by multi-configuration ab innio methods, providing better estimates of the exchange constants