Benchmark Study of the Structural and Thermochemical Properties of a Dihydroazulene/Vinylheptafulvene Photoswitch.

We investigate the performance of four different density functional theory (DFT) functionals (M06-2X, ωB97X-D, PBE0, and B3LYP-D3BJ) for calculating the structural and thermochemical properties of the dihydroazulene/vinylheptafulvene photoswitch (DHA/VHF). We find that all the tested DFT functionals yield equilibrium geometries in good agreement with higher level CCSD/cc-pVDZ calculations and that the basis set had little influence on the geometries of the photoswitch. We found a negligible difference in the thermal contribution to the Gibbs free energy between the tested functionals, indicating that the largest source of error when calculating storage free energies originates from errors in the calculated single point energies. It was found that ωB97X-D and M06-2X performed decently for predicting storage energies. While B3LYP-D3BJ and PBE0 generally underestimated the storage energy compared to CCSD(T)-F12a/VDZ-F12 results. Therefore, we tested if domain based local pair natural orbital coupled-cluster (DLPNO-CCSD(T)) provided an improvement over density functional theory methods for the single point energies. We observed that the DLPNO-CCSD(T) storage energies were in better agreement with CCSD(T)-F12a/VDZ-F12 results than the DFT results. The DLPNO-CCSD(T) results already converged at cc-pVTZ quality basis set, making it possible to perform accurate estimates of the thermochemical properties in a time frame that makes the DLPNO-CCSD(T) method feasible for routine calculations on the photoswitch. Using DLPNO-CCSD(T)/cc-pVTZ, we calculate accurate storage energies for currently synthesized derivatives of the DHA/VHF photoswitch.