A comparison of the electronic transition energies for ethene, isobutene, formaldehyde, and acetone calculated using RPA, TDDFT, and EOM-CCSD. Effect of basis sets

The effect of basis sets on the calculated transition energies for ethene. formaldehyde, acetone, and isobutene has been studied at the RPA, TDDFT. and EOM-CCSD theoretical levels. Polarization functions beyond the (d,p) level have little effect on the calculated energies. However, diffuse functions have a major effect on the calculated energies. Using 6-311(2+,2+)G(d,p) which has two sets of diffuse functions, EOM-CCSD gave very good agreement with the available experimental data in most cases. Another set of diffuse functions led to lower transition energies in a few cases. The RPA calculations for ethene are in fairly good agreement with the first 10 experimental transition energies, and TDDF`T is less satisfactory. On the other hand, for formaldehyde and acetone, TDDFT gives fairly good agreement with experiment for the first states, and RPA is quite unsatisfactory. The use of global diffuse functions instead of additional atom-centered diffuse functions was examined. They proved to be quite satisfactory for the compounds in this investigation and serve to reduce the time required for the calculations.