Assessment of mixed basis set and ONIOM methods on the activation energy of ring opening reactions of substituted cyclobutenes

The performance of the mixed basis set and the ONIOM methods on the activation energy of the ring opening pericyclic reaction of substituted cyclobutenes was reported. The B3LYP, BB1K and MPWB1K functionals with several basis sets were used in this investigation. The study indicates that the B3LYP functional is more appropriate for predicting the activation energy of the ring opening reaction than the BB1K and MPWB1K functionals. However, the BB1K and MPWB1K functionals can represent the substituent effect on the activation energy as accurately as the B3LYP functional. In general, the mixed basis set yields less root mean square error but takes significantly more computational time than the ONIOM method. The results suggest that the 6-31G(d):CEP-31G mixed basis set is the method of choice for this system judging from the least root mean square error and the least maximum absolute error, whereas the MPWB1K functional with the 6-311+G(2d,2p):6-31G(d) mixed basis set is the method of choice for calculating the relative activation energy.