title: Pragmatic ab initio prediction of enthalpies of formation for large molecules: accuracy of MP2 geometries and frequencies using CCSD(T) correlation energies.
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.
authors.affiliation: Quantum Theory Project, Gainesville, FL 32611, USA. r.molt.chemical.physics@gmail.com
authors.forename: Robert W
authors.initials: RW
authors.lastname: Molt
authors.forename: Alexandre
authors.initials: A
authors.lastname: Bazanté
authors.forename: Thomas
authors.initials: T
authors.lastname: Watson
authors.forename: Rodney J
authors.initials: RJ
authors.lastname: Bartlett
mesh_terms: D008958:Models, Molecular; D011789:Quantum Theory; D013816:Thermodynamics