Low-lying π∗ resonances associated with cyano groups: A CAP/SAC-CI study

Abstract The complex absorbing potential (CAP)/symmetry-adapted cluster-configuration interaction (SAC-CI) method is applied to low-lying π ∗ resonance states of molecules containing one or two cyano (CN) groups. Benchmark calculations are carried out comparing the non-variational and approximate variational approach of SAC-CI and studying the selection threshold of operators. Experimental resonance positions from electron transmission spectroscopy (ETS) are reproduced provided the anticipated deviations due to vibronic effects are taken into account. Moreover, the calculated positions and widths agree well with those obtained in previous electron scattering calculations for HCN, CH 3 CN and their isonitriles. Based on our results, we suggest a reassignment of the experimental ETS of fumaronitrile and malononitrile. Our present results demonstrate again that the CAP/SAC-CI method reliably predicts low-lying π ∗ resonances, and regarding the total numbers of molecules and resonances investigated, it is fair to say that it is presently the most extensively used high-level method in the temporary anion field.