Combination of a Voronoi-Type Complex Absorbing Potential with the XMS-CASPT2 Method and Pilot Applications

Electronic resonances are metastable (N + 1) electron states, in other words, discrete states embedded in an electronic continuum. While great progress has been made for certain types of resonances-for example, temporary anions created by attaching one excess electron to a closed shell neutral-resonances in general remain a great challenge of quantum chemistry because a successful description of the decay requires a balanced description of the bound and continuum aspect of the resonance. Here, a smoothed Voronoi complex absorbing potential (CAP) is combined with the XMS-CASPT2 method, which enables us to address the balance challenge by appropriate choice of the CAS space. To reduce the computational cost, the method is implemented in the projected scheme. In this pilot application, three temporary anions serve as benchmarks: the π* resonance state of formaldehyde; the π* and σ* resonance states of chloroethene as functions of the C-Cl bond dissociation coordinate; and the ⁴Π and ²Π resonance states of N₂^-. The convergence of the CAP/XMS-CASPT2 results has been systematically examined with respect to the size of the active space. Resonance parameters predicted by the CAP/XMS-CASPT2 method agree well with CAP/SAC-CI results (deviations of about 0.15 eV); however, as expected, CAP/XMS-CASPT2 has clear advantages in the bond dissociation region. The advantages of CAP/XMS-CASPT2 are further demonstrated in the calculations of ⁴Π and ²Π resonance states of N₂^- including their ³Σ⁺ and ³Δ parent states. Three of the involved states (²Π, ³Σ⁺, and ³Δ) possess multireference character, and CAP/XMS-CASPT2 can easily describe these states with a relatively modest active space.