Negative Ion Resonance States: The Fock-Space Coupled-Cluster Way

Abstract

The negative ion resonance states, which are electronmolecule metastable compound states, play the most important role in free-electron controlled molecular reactions and low-energy free-electroninduced DNA damage. Their electronic structure is often only poorly described but crucial to an understanding of their reaction dynamics. One of the most important challenges to current electronic structure theory is the computation of negative ion resonance states. As a major step forward, coupled-cluster theories, which are well-known for their ability to produce the best approximate bound state electronic eigen solutions, are upgraded to offer the most accurate and effective approximations for negative ion resonance states. The existing Fock-space coupled-cluster (FSCC) and the equation-ofmotion coupled-cluster (EOM-CC) approaches that compute bound states are redesigned for the direct and simultaneous determination of both the kinetic energy of the free electron at which the electron-molecule compound states are resonantly formed and the corresponding autodetachment decay rate of the electron from the metastable compound state. This Feature Article reviews the computation of negative ion resonances using the FSCC approach and, in passing, provides the highlights of the equivalent EOM-CC approach.

Lifetimes of Be32– and Mg32– Cluster Dianions

“…Currently, three continuum methods are widely used: the complex absorbing potential (CAP) method, − the regularized analytical continuation (RAC) method, , and the Hazi–Taylor stabilization method. , While these three methods differ greatly in their basic ideas and underlying theory, they share the common feature of being two-step compound methods. In step 1, the electronic Hamiltonian is parametrized and ab initio calculations are performed for many values of the parameter.…”

Projected Complex Absorbing Potential Multireference Configuration Interaction Approach for Shape and Feshbach Resonances

“…The success of CAP based methods can be attributed to the ease of integration with any available electronic structure method. Methods augmented with CAP include coupled cluster (EOM-CC and FSMRCC), algebraic diagrammatic construction (ADC) schemes, symmetry adapted cluster/configuration interaction (SAC-CI), multireference configuration interaction (MRCI), multireference perturbation theory (XMCQDPT2, XMS-CASPT2), electronic propagator (EP), and density functional theory (DFT). − Several properties such as complex Dyson orbitals and natural transition orbitals, as well as analytic gradients, and algorithms for locating crossing points have also been developed within CAP. − Recent implementation of analytic gradients within the CAP-EOM-CC formalism has enabled computation of optimized structures and exceptional points. − Optimized structures and adiabatic electron affinities have been computed for several polyatomic systems such as formaldehyde, formic acid, and ethylene anions using the CAP-EOM-CC formalism . Recently, CAP-EOM-CC has also been employed to compute minimum energy exceptional points on crossings between resonances and minimum energy crossing points between neutral and anionic surfaces. , The routine application of a variety of CAP augmented methods to compute resonances serves as a testament to their success compared to other non-Hermitian or even Hermitian methods.…”

Three-Body Excitations in Fock-Space Coupled-Cluster: Fourth Order Perturbation Correction to Electron Affinity and Its Relation to Bondonic Formalism

“…The Fock-space-based approach, to correlate model spaces that have different numbers of electrons, also has the advantage of the calculation of EA in a direct manner. Comprehensive discussions on FSCC are presented in the references [40,45,46]. The FSCC, in its singles and doubles model (FSCCSD), has been well developed and studied for direct difference energies [40][41][42], as well as for energy derivatives, by Pal and co-workers [47][48][49].…”

“…Their approach is not only applicable for complete model spaces, but it is also applicable for incomplete model spaces. Moreover, the implementations of the complex absorbing potential (CAP) in the FSMRCC framework for resonant states, have also been achieved by Pal and co-workers [46,[50][51][52].…”