Assessment of Various Electronic Structure Methods for Characterizing Temporary Anion States: Application to the Ground State Anions of N2, C2H2, C2H4, and C6H6

The theoretical characterization of temporary anions is an especially challenging problem. In the present study we assess the performance of several electronic structure methods when used in conjunction with the stabilization method to characterize temporary anion states. The ground state anions of N-2, C2H2, C2H4, and C6H6 are used as the test systems, with the most extensive testing being done for N-2. For the (2)Pi(g) anion state of N-2(-) the ADC(2), EOM-MP2, and EOM-CCSD methods give values of the resonance parameters in excellent agreement with the results of prior high-level calculations. For the hydrocarbon systems, the EOM-MP2 method consistently provides excellent agreement with the EOM-CCSD results for the test systems, whereas the ADC(2) considerably underestimates the widths for ethylene and benzene. Several density functional theory (DFT) approaches are tested and, although none performs as well as the EOM-MP2 method, it is found that inclusion of Hartree-Fock exchange greatly improves the results. Of the DFT-based methods, time-dependent DFT with standard correlation finictionals and use of Hartree-Fock exchange provides the best performance for N-2(-) over the range of bond lengths considered and is also found to give reasonable values of the resonance parameters of the three hydrocarbon molecules.