A systematic comparison of density functional methods for determining spin-state energy gaps and spin transition temperature of spin crossover complexes.

In this paper, the spin-crossover (SCO) behavior of 26 transition metal complexes has been investigated by Density Functional Theory (DFT) using nine density functionals such as TPSS, BLYP, TPSSh, B3LYP, B3LYP*, OPBE, O3LYP, B3P86, and X3LYP, providing a comprehensive analysis of their effect towards computing spin-state energy gaps and spin transition temperature (T 1/2) of these SCO complexes. The SCO behavior of a complex highly depends on the free energy balance of high- and low-spin states, which is likewise influenced by physical properties including dispersion and vibrational entropy, which are all considered in the performed DFT calculations. Among all the tested functionals, the hybrid meta-GGA TPSSh functional and the B3LYP* functional predict the correct ground state and a reasonable HS-LS energy gap for all the SCO complexes. Their contemporary functionals, such as pure meta-GGA TPSS and GGA BLYP functionals, also predict the correct GS for all the complexes, but they overestimate the HS-LS gaps. Interestingly, the OPBE and the B3P86 functionals also predict the correct ground state for nearly 50% of the studied complexes. The TPSSh-predicted energy gap is in the proper range for SCO to occur in the majority of cases, including the SCO complexes with unusual geometry, and the T 1/2 predicted using this functional is also in good agreement with the experimental ones.