Local Wave Function\nEmbedding: Correlation Regions\nin PNO-LCCSD(T)-F12 Calculations

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