On the applicability of the MP2.5 approximation for open-shell systems. Case study of atmospheric reactivity.

• MP2.5/cc-pvtz successfully applied to reaction energies including halogenated hydrocarbons and open-shell systems. • No significant difference between UHF- and ROHF-reference of MP2.5. • Simple a priori diagnostics based on the analysis of the first few CCSD/cc-pvdz excitation amplitudes. • Favorable correlation between MP2.5 and CCSD(T) reaction energies. • Promising potential of MP2.5 for chemical reactivity of very large systems (e.g., clusters). The performance of the MP2.5 method (MP2 augmented by scaled MP3 correction) is tested for the set of atmospheric reactions including both closed- and open-shell species. The reactions include series of halogen substituted hydrocarbons CH n X 4-n (n = 1–3, X = Cl, Br, I) reacting with OH radical. Both the UHF and ROHF reference wave functions for the open-shell molecules are considered. MP2/cc-pVTZ optimized geometries were used for the calculation of the reaction energies. The accuracy of MP2.5 is discussed in terms of differences between MP2.5/cc-pVTZ and CCSD(T)/cc-pVTZ energies. Simple a priori diagnostic based on the analysis of the coupled-cluster excitation amplitudes calculated at lower CCSD/cc-pVDZ level is introduced. The method seems to be a promising extension of MP2.5 from the realm of intermolecular interactions to chemical reactivity. [ABSTRACT FROM AUTHOR]