G4-SP, G4(MP2)-SP, G4-sc, and G4(MP2)-sc: Modifications to G4 and G4(MP2) for the Treatment of Medium-Sized Radicals

Bun Chan; Michelle L Coote; Leo Radom

doi:10.1021/ct100266u

G4-SP, G4(MP2)-SP, G4-sc, and G4(MP2)-sc: Modifications to G4 and G4(MP2) for the Treatment of Medium-Sized Radicals

J Chem Theory Comput. 2010 Sep 14;6(9):2647-53. doi: 10.1021/ct100266u. Epub 2010 Aug 19.

Authors

Bun Chan¹, Michelle L Coote¹, Leo Radom¹

Affiliation

¹ ARC Center of Excellence for Free Radical Chemistry and Biotechnology, School of Chemistry, University of Sydney, NSW 2006, Australia, and Research School of Chemistry, Australian National University, ACT 0200, Australia.

PMID: 26616066
DOI: 10.1021/ct100266u

Abstract

The G4-SP and G4(MP2)-SP procedures are introduced, as alternatives to G4 and G4(MP2), to overcome shortcomings associated with the treatment of spin polarization (SP) in large open-shell systems. The new methods employ a converging SP term, to replace the diverging A' treatment used in the G4 and G4(MP2) formulations. The G4-SP and G4(MP2)-SP procedures have mean absolute deviations (MADs) from experimental energies of 3.49 and 4.37 kJ mol(-1), respectively, for the G3/05 test set, which are comparable to the MAD values for G4 and G4(MP2) but eliminate the problem of a diverging A' term. For energies involving larger radicals, G4(MP2)-SP performs better than standard G4(MP2). Alternative methods, including G4-5H, G4(MP2)-5H, G4-sc, and G4(MP2)-sc, are also introduced to avoid the problem of an indefinitely increasing SP correction in standard G4 or G4(MP2) for reactions involving larger open-shell systems.