Analysis and reporting recommendations for theoretical and experimental ionization potentials based on the study of 53 medium sized molecules using the IP-EOMCCSD method.

The precision and accuracy of theoretical vertical ionization potential calculations has improved to the point where more care is needed to make valid comparisons with experimental measurements then is currently the norm. Vertical ionization potentials (IPs) computed using the IP-EOMCCSD method are reported for 53 medium sized molecules (6 – 32 atoms) and compared with statistically evaluated experimental vertical IPs. Based on this comparison, theoretical IPs should be extrapolated to the complete basis set limit and corrected for vibrational zero-point energy, while for experimental data the intensity weighted mean band position should be reported as the vertical IP. Experimental data available for ethylene, E-2-butene, 2,5-dihydrofuran and pyrrole were re-analyzed and compared with zero-point energy corrected complete basis set theoretical estimates, yielding an average discrepancy of 0.05 eV between theory and experiment. In contrast the average of reported experimental vertical IPs (the comparison usually made) yielded an average discrepancy of 0.25 eV between theory and experiment for these molecules. Further analysis of the remaining molecules in the data set suggests that the majority of reported experimental vertical IPs are low because band asymmetries were not accounted for when assigning IP values. This leads to fortuitous good agreement between experiment and computations using the smaller aug-cc-pVDZ basis set without zero-point correction. In the case of 1,4-cyclohexadiene there is strong evidence for experimental uncertainty accounting for the discrepency between theory and experiment. The presented results provide a benchmark for evaluating both experimental and theoretical estimates of vertical ionization potentials for the 53 molecules studied.