Incremental scheme for high level electronic correlation calculations of large molecular clusters

An efficient and accurate approach inc2-db-B0 for applying high level ab initio correlation methods CCSD(T) and CCSD(T)-F12 to large molecular clusters is proposed. It exploits the locality of the electron correlation and only requires the incremental expansion of the correlation energy up to second order, thus significantly increasing the calculation speed. Calibration calculations for water clusters with up to 20 molecules yield errors in total CCSD(T) energies of about 0.15 kcal/mol per molecule. The approach is further examined for three systems: (H2O)17, (CH3CN)6 and (CH2=CH2)13. All have isomers very close in energy and their polarised triple-zeta basis set CCSD(T) calculations involve between 1200 and 2400 basis functions. The reported inc2-db-B0 calculations are feasible for ordinary hardware and provide relative energies with errors of less than 0.15 kcal/mol in comparison to the standard implementation, whereas the computational cost is reduced by more than 100 times. When comparing the results of selected modern density functional theory (DFT) approaches to the ab initio reference data presented here, we observe that the accuracy of DFT is still somewhat unpredictable for the small energy differences between the cluster isomers.