Large-scale parallel calculations with combined coupled cluster and molecular mechanics formalism: Excitation energies of zinc–porphyrin in aqueous solution

The need for combined QM/MM approaches with high-level coupled cluster (CC) methods in the QM core is unquestionable. In an effort to make these methods widely applicable, the scalability of CC approaches across hundreds/thousands of CPUs will play a central role. In this Letter we report the performance of our CC codes in calculations of vertical excitation energies of low-lying excited states of the zinc–porphyrin molecule in aqueous solution. Coupled cluster description of excited states is based on equation-of-motion approach with singles and doubles (EOMCCSD) as well as its non-iterative extension for triply excited configurations. These results are compared with those obtained with time-dependent density functional theory (TD-DFT).