Benchmarking the Approximate Second-Order Coupled-Cluster Method on Biochromophores.

Extensive benchmarking calculations are presented to assess the accuracy of commonly used quantum chemical methods in studying excited state properties of biochromophores. The first few excited states of 12 common model chromophores of photoactive yellow protein, green fluorescent protein, and rhodopsin have been studied using approximate second-order coupled-cluster (CC2) and linear-response time-dependent density functional theory (TDDFT) calculations. The study comprises investigations of basis-set dependences on CC2 excitation energies as well as comparisons of the CC2 results with excitation energies obtained at other computational levels and with experimental data. The basis-set study shows that the accuracy of the two lowest excitation energies is generally sufficient when triple-ζ basis sets augmented with polarization functions are employed, whereas the third and higher excited states were found to require diffuse basis functions in the basis set. Augmenting the basis set with diffuse functions contributes less than 0.15 eV to the excitation energies of low-lying excited states, except for some of the studied anionic states and for Rydberg states. Calculations at the TDDFT level using the B3LYP functional show the necessity of stabilizing anions with point charges or counterions when aiming at reliable electronic excitation spectra. The two lowest excitation energies of the green fluorescent protein and rhodopsin chromophores calculated at the CC2 level agree within 0.15 eV with experimental excitation energies, whereas the B3LYP values are somewhat less accurate, with a maximum deviation of 0.27 eV. The computed excitation energies for the photoactive yellow protein chromophore models deviate from available experimental values by 0.3-0.4 eV and 0.1-0.5 eV, at the CC2 and B3LYP levels of theory, respectively.