Abstract
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.
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