MP2-F12 Basis Set Convergence near the Complete Basis Set Limit: Are Functions Sufficient?

We have investigated the title question for the W4-08 thermochemical benchmark using -saturated truncations of a large reference (REF) basis set, as well as for standard F12-optimized basis sets. With the REF basis set, the root-mean-square (RMS) contribution of functions to the MP2-F12 total atomization energies (TAEs) is about 0.01 kcal/mol, the largest individual contributions being 0.04 kcal/mol for P and P. However, even for these cases, basis set extrapolation from {,} basis sets adequately addresses the problem. Using basis sets insufficiently saturated in the angular momenta may lead to exaggerated function contributions. For extrapolation from and basis sets, basis set convergence appears to be quite close to the theoretical asymptotic ∝ behavior. We hence conclude that functions are sufficient even for highly demanding F12 applications. With one-parameter extrapolation, and basis sets are adequate, aug-cc-pV{T,Q}Z-F12 yielding a RMSD = 0.03 kcal/mol. A limited exploration of CCSD(F12*) and CCSD-F12b suggests our conclusions are applicable to higher-level F12 methods as well.