Extrapolation to the Gold-Standard in Quantum Chemistry: Computationally Efficient and Accurate CCSD(T) Energies for Large Molecules Using an Automated Thermochemical Hierarchy.

The CCSD(T) method is known as the gold-standard in quantum chemistry and has been the method of choice in quantum chemistry for over 20 years to obtain accurate bond energies and molecular properties. Its computational cost formally scales as the seventh power of the size of the system and can be prohibitive for large molecules. As part of our efforts to reduce the computational cost of the CCSD(T) method yet retain its accuracy, we present a simple, efficient, and user-friendly protocol to extrapolate to CCSD(T) energies in conjunction with MP2 energies. The method is based on the automated error-canceling thermochemical hierarchy previously developed by us called the Connectivity-Based Hierarchy (CBH). For a test set containing 30 diverse nonaromatic organic molecules and biomonomers, we obtain highly accurate extrapolated CCSD(T) energies (with a mean absolute error of only 0.2-0.3 kcal/mol with different basis-set). Additionally, the work also features the successful extrapolation to CCSD energies using a similar protocol.