Accuracy of atomization energies and reaction enthalpies in standard and extrapolated electronic wave function/basis set calculations

The accuracy of standard ab initio wave-function calculations of atomizatio n energies and reaction enthalpies has been assessed by comparing with expe rimental data for 16 small closed-shell molecules and 13 isogyric reactions . The investigated wave-function models are Hartree-Fock (HF), Moller-Pless et second-order perturbation theory (MP2), coupled-cluster theory with sing les and doubles excitations (CCSD) and CCSD with perturbative triple-excita tion corrections [CCSD(T)]; the one-electron basis sets used are the correl ation-consistent cc-pVxZ and cc-pCVxZ basis sets with cardinal numbers x=D, T, Q, 5, and 6. Results close to the basis-set limit have been obtained by using two-point extrapolations. In agreement with previous studies, it is found that the intrinsic error of the CCSD(T) method is less than chemical accuracy (approximate to 4 kJ/mol) for both atomization energies and reacti on enthalpies. The mean and maximum absolute errors of the best CCSD(T) cal culations are 0.8 and 2.3 kJ/mol for the atomization energies and 1.0 and 2 .3 kJ/mol for the reaction enthalpies. Chemical accuracy is obtained alread y from the extrapolations based on the cc-pCVTZ and cc-pCVQZ basis sets-wit h mean and maximum absolute errors of 1.7 and 4.0 kJ/mol for atomization en ergies and 1.3 and 3.1 kJ/mol for reaction enthalpies. The intrinsic errors of the Hartree-Fock, MP2, and CCSD wave-function models are significantly larger than for CCSD(T). For CCSD and MP2, the mean absolute errors in the basis set limit are about 32 kJ/mol for the atomization energies and about 10 and 15 kJ/mol, respectively, for the reaction enthalpies. For the Hartre e-Fock model, the mean absolute errors are 405 and 29 kJ/mol for atomizatio n energies and reaction enthalpies, respectively. Correlation of the core e lectrons is important in order to obtain accurate results with CCSD(T). Wit hout compromising the accuracy, the core contribution may be calculated wit h a basis set that has one cardinal number lower than that used for the val ence correlation contribution. Basis-set extrapolation should be used for b oth the core and the valence contributions. (C) 2000 American Institute of Physics. [S0021- 9606(00)30119-2].