Predicting critical temperatures of iron(II) spin crossover materials: Density functional theory plus U approach

A first-principles study of critical temperatures (T-c) of spin crossover (SCO) materials requires accurate description of the strongly correlated 3d electrons as well as much computational effort. This task is still a challenge for the widely used local density or generalized gradient approximations (LDA/GGA) and hybrid functionals. One remedy, termed density functional theory plus U (DFT+U) approach, introduces a Hubbard U term to deal with the localized electrons at marginal computational cost, while treats the delocalized electrons with LDA/GGA. Here, we employ the DFT+ U approach to investigate the T-c of a pair of iron(II) SCO molecular crystals (alpha and beta phase), where identical constituent molecules are packed in different ways. We first calculate the adiabatic high spin-low spin energy splitting Delta E-HL and molecular vibrational frequencies in both spin states, then obtain the temperature dependent enthalpy and entropy changes (Delta H and Delta S), and finally extract T-c by exploiting the Delta H/T - T and Delta S - T relationships. The results are in agreement with experiment. Analysis of geometries and electronic structures shows that the local ligand field in the a phase is slightly weakened by the H-bondings involving the ligand atoms and the specific crystal packing style. We find that this effect is largely responsible for the difference in T-c of the two phases. This study shows the applicability of the DFT+ U approach for predicting T-c of SCO materials, and provides a clear insight into the subtle influence of the crystal packing effects on SCO behavior. (C) 2014 AIP Publishing LLC.