Pragmatic ab initio prediction of enthalpies of formation for large molecules: accuracy of MP2 geometries and frequencies using CCSD(T) correlation energies.

We have addressed the accuracy of calculating the enthalpy of formation of an arbitrary single reference molecule using practical ab initio methodologies. It is known that MP2 geometries with a triple zeta basis set are almost as reliable as CCSD(T) geometries. It is also known that CCSD(T) correlation energies, with basis extrapolation, feature chemical accuracy for single-reference molecules. We investigate what accuracy one might expect in enthalpies of formation from a MP2 geometry, MP2 harmonic vibrational frequencies, a CCSD(T) correlation energy using triple zeta basis sets. It is far from obvious, a priori, as to which error source contributes most significantly. We observe that the accuracy in calculating enthalpies of formation of single-reference molecules with this protocol is 4 kcal mol(-1); our error analysis shows this comes almost exclusively from the correlation energy basis extrapolation, rather than errors intrinsic to MP2.