Benchmarking quantum chemistry methods in calculations of electronic excitations

Quantum chemistry methods are applied to obtain numerical solutions of the Schr¨odinger equation for molecular systems. Calculations of transitions between electronic states of large molecules present one of the greatest challenges in this field which require the use of supercomputer resources. In this work we describe the results of benchmark calculations of electronic excitation in the protein domains which were designed to engineer novel fluorescent markers operating in the near-infrared region. We demonstrate that such complex systems can be efficiently modeled with the hybrid qunatum mechanics/molecular mechanics approach (QM/MM) using the modern supercomputers. More specifically, the time-dependent density functional theory (TD-DFT) method was primarily tested with respect to its performance and accuracy. GAMESS (US) and NWChem software were benchmarked in direct and storage-based TDDFT calculations with the hybrid B3LYP density functional, both showing good scaling up to 32 nodes. We note that conventional SCF calculations greatly outperform direct SCF calculations for our test system. Accuracy of TD-DFT excitation energies was estimated by a comparison to the more accurate ab initio XMCQDPT2 method.