Local Wave Function Embedding: Correlation Regions in PNO-LCCSD(T)-F12 Calculations

Many chemical reactions
affect only a rather small number of bonds,
leaving the largest part of the chemical and geometrical structure
of the molecules nearly unchanged. In this work we extended the previously
proposed region method [J. Chem. Phys. 128, 144106 (2008)] to PNO-LCCSD(T)-F12.
Using this method, we investigate whether accurate reaction energies
for larger systems can be obtained by correlating only the electrons
in a region of localized molecular orbitals close to the reaction
center at high-level (PNO-LCCSD(T)-F12). The remainder is either treated
at lower level (PNO-LMP2-F12) or left uncorrelated (Hartree–Fock
frozen core). It is demonstrated that indeed the computed reaction
energies converge rather quickly with the size of the correlation
regions toward the results of the full calculations. Typically, 2–3
bonds from the reacting atoms need to be included to reproduce the
results of the full calculations to within ±0.2 kcal/mol. We
also computed spin-state energy differences in a large transition
metal complex, where a factor of 15 in computation time could be saved,
still yielding a result that is within ±0.1 kcal/mol of the one
obtained in a full PNO-LCCSD(T)-F12 calculation.