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

Abstract

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 u and 2Π u 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 u and 2Π u resonance states of N2 – including their 3Σ u + and 3Δ u parent states. Three of the involved states (2Π u , 3Σ u +, and 3Δ u ) possess multireference character, and CAP/XMS-CASPT2 can easily describe these states with a relatively modest active space.

Lifetimes of Be32– and Mg32– Cluster Dianions

“…In other words, the electronic structure method must be size-extensive and internally size-extensive (cf. introduction of ref ).…”

Elektronische Resonanzzustände – warum die Energie komplex sein kann, Trendbericht Theoretische Chemie 2020, Teil 1

“…27,28) Zudem existieren unter anderem Implementierungen für CI-Methoden [29][30][31] und seit dem Jahr 2017 auch für störungstheoretische Multireferenzmethoden. 32,33) Letztgenannte eignen sich insbesondere für Feshbach-Resonanzen und um Bindungsbrüche zu beschreiben.…”

Features of the Theory of Resonant Electron Scattering on Atoms

“…A fairly complete list of articles devoted to calculations of many-electron systems by the CAP method can be found in [12]. Among the works that were not included in it, we should note the work [13] and the methodological works [14][15][16][17][18][19][20] and reviews [21,22].…”