The correlation Consistent composite Approach: The spin contamination effect on an MP2-based composite methodology

• Published in: Chemical physics letters 2011-02, Vol.504 (1-3), p.88-94
• Main Authors: Williams, T. Gavin, DeYonker, Nathan J., Ho, Bryan S., Wilson, Angela K.
• Format: Article
• Language: English
• ISSN: 0009-2614; 1873-4448
• DOI: 10.1016/j.cplett.2011.01.020

UHF-based correlation consistent composite approach (ccCA) has a statistical bias for molecules which show significant spin contamination. The ccCA methodology is examined and reformulated to increase the accuracy for radicals. [Display omitted] ► A restricted open-shell reference wavefunction composite approach is introduced. ► RO-ccCA removes spin contamination. ► Substantial improvement in energetic prediction occurs for open-shell systems. The correlation consistent composite approach (ccCA) is reformulated utilizing a restricted open-shell reference wavefunction to remove spin-contamination from open-shell systems. The RO-ccCA-PS3 variant has an MAD of 0.65kcalmol−1 for the 32 open-shell systems in the G3/99 enthalpy of formation test set, as compared to 0.77kcalmol−1 using the unrestricted formulation. For the 22 most highly spin-contaminated species in the G3/99 test set, RO-ccCA produces a lower MAD; 1.20 versus 2.17kcalmol−1. Additionally, the performance of RO-ccCA is examined for 30 bond dissociation energies and found to show improved accuracy over unrestricted ccCA.