Mountaineering Strategy to Excited States: Highly Accurate Energies and Benchmarks for Exotic Molecules and Radicals.

Aiming at completing the sets of FCI-quality transition energies that we recently developed (J. Chem. Theory Comput. 2018, 14, 4360-4379, ibid. 2019, 15, 1939-1956, and ibid. 2020, 16, 1711-1741), we provide, in the present contribution, ultra-accurate vertical excitation energies for a series of "exotic" closed-shell molecules containing F, Cl, P, and Si atoms and small radicals, such as CON and its variants, that were not considered to date in such investigations. This represents a total of 81 high-quality transitions obtained with a series of diffuse-containing basis sets of various sizes. For the exotic compounds, these transitions are used to perform benchmarks with a vast array of lower level models, i.e., CIS(D), EOM-MP2, (SOS/SCS)-CC2, STEOM-CCSD, CCSD, CCSDR(3), CCSDT-3, (SOS-)ADC(2), and ADC(3). Additional comparisons are made with literature data. For the open-shell compounds, we compared the performance of both the unrestricted and the restricted open-shell CCSD and CC3 formalisms.