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 (<i>T</i> _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 <i>meta</i>-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 <i>meta</i>-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 <i>T</i> _1/2 predicted using this functional is also in good agreement with the experimental ones.