Accurate thermochemistry at affordable cost by means of an improved version of the JunChS‐F12 model chemistry

The junChS‐F12 composite method has been improved by means of the latest implementation of the CCSD(F12*)(T+) ansatz and validated for the thermochemistry of molecules containing atoms of the first three rows of the periodic table. A thorough benchmark showed that this model, in conjunction with cost‐effective revDSD‐PBEP86‐D3(BJ) reference geometries, offers an optimal compromise between accuracy and computational cost. If improved geometries are sought, the most effective option is to add MP2‐F12 core‐valence correlation corrections to CCSD(T)‐F12b/jun‐cc‐pVTZ geometries without the need of performing any extrapolation to the complete basis set limit. In the same vein, CCSD(T)‐F12b/jun‐cc‐pVTZ harmonic frequencies are remarkably accurate without any additional contribution. Pilot applications to noncovalent intermolecular interactions, conformational landscapes, and tautomeric equilibria confirm the effectiveness and reliability of the model. A new variant of the junChS‐F12 composite method has been introduced, which employs a reduced‐cost implementation of the accurate and size‐consistent CCSD(F12*)(T+) ansatz. The new model has been validated by applications to thermochemistry, noncovalent complexes, conformational landscapes, and tautomeric equilibria.