Benchmarking Molecular and Periodic Extended Tight Binding Methods for Spin-State Energies in Spin-Crossover Systems

The calculation of energy differences between states is a significant challenge for electronic structure methods. The energy differences of the spin states in spin crossover compounds are very small, which makes them one of the most challenging systems to calculate, especially when compared to systems with transition metals that do not have spin transitions. The problem is aggravated if we also want to extend it to periodic systems, which is how they are usually characterized experimentally. This fact significantly restricts the methods that can be employed. In this paper, we have used extended tight binding to calculate the high- and low-spin energy differences for a test case of twenty-three spin crossover systems by performing molecular and periodic calculations. Although the results are not as accurate as those obtained with some DFT functionals, the computational time and resources required make them an excellent option for fast results and handling complex systems, such as large molecules or unit cells with many molecules.