Evaluation of the IP-EOMCCSD Method for Estimating Experimental Ionization Potentials Using 53 Medium Sized Molecules

Vertical ionization potentials (IPs) computed using the IP-EOMCCSD method are reported for 53 medium sized molecules (6 – 32 atoms) and compared with average experimental vertical IPs. The calculations are practical on a modest computational cluster and yield good agreement with experimental values using the aug-cc-pVDZ basis set, with an average deviation from the experimental IP of −0.04 eV. The accuracy of IP computations appears to be approaching the point where possible systematic experimental errors can be identified. Although good extrapolations to the complete basis set limit for the IP are achievable using just the aug-cc-pVDZ and aug-cc-pVTZ basis sets, deviations of the extrapolation from experimental values suggest that inclusion of higher order "triples" may make the computational method more broadly applicable. Examination of experimental spectra for ethylene, E-2-butene, 2,5-dihydrofuran and pyrrole reinforces the observations of Davidson and Jarzęcki1 that experimental vertical IPs are usually extracted from experimental data in a manner that does not account for band asymmetries, making direct comparison to computations difficult. Despite the good agreement with experiment when using the aug-cc-pVDZ basis set, for the molecules investigated most of these reported experimental IPs are below the actual value, likely by no more than 0.4 eV. This set of 53 molecules is recommended as a benchmark comparison set for computational and experimental IP results.