No abstract

<div> <div> <div>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.<br></div> </div> </div>

<div> <div> <div>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. </div> </div> </div>

Comparison of statistically evaluated experimental vertical ionization energies (IEs) for 53 medium‐sized molecules (6‐34 atoms) with ionization potential equation‐of‐motion coupled‐cluster with singles and doubles (IP‐EOMCCSD) computations shows that discrepancies between computed and experimental results can be accounted for with a combination of experimental and theoretical contributions. Discrepancies can be minimized by extrapolating computations to the complete basis set limit and correcting for vibrational zero‐point energy (ZPE) while comparing with experimental IEs calculated as the intensity‐weighted mean band position to account for band asymmetries. This procedure reduced the average discrepancy for ethylene, (E)‐2‐butene, 2,5‐dihydrofuran, and pyrrole from 0.25 to 0.05 eV. Agreement between reported vertical IEs and computations without either making adjustments as described in this paper or using complete simulation of the ionization spectrum should be considered fortuitous. The comparisons made in this work show that estimates of vertical and adiabatic IE made using IP‐EOMCCSD extrapolated to the complete basis set limit and corrected for vibrational ZPE can be used with reasonable confidence when experimental values are not available.

<div> <div> <div>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. </div> </div> </div>