Extending Quantum Chemistry of Bound States to Electronic Resonances

Abstract

Electronic resonances are metastable states with finite lifetime embedded in the ionization or detachment continuum. They are ubiquitous in chemistry, physics, and biology. Resonances play a central role in processes as diverse as DNA radiolysis, plasmonic catalysis, and attosecond spectroscopy. This review describes novel equation-of-motion coupled-cluster (EOM-CC) methods designed to treat resonances and bound states on an equal footing. Built on complex-variable techniques such as complex scaling and complex absorbing potentials that allow resonances to be associated with a single eigenstate of the molecular Hamiltonian rather than several continuum eigenstates, these methods extend electronic-structure tools developed for bound states to electronic resonances. Selected examples emphasize the formal advantages as well as the numerical accuracy of EOM-CC in the treatment of electronic resonances. Connections to experimental observables such as spectra and cross sections, as well as practical aspects of implementing complex-valued approaches, are also discussed.

Efficient calculations of a large number of highly excited states for multiconfigurational wavefunctions

“…However, with Hermitian mechanics and a finite atom‐centered basis set, it is challenging to describe the continuum and thus the coupling. Solutions using complex‐valued approaches exist but are associated with increased cost . Simply neglecting the continuum coupling altogether using CVS gives good results and is better than using a poor continuum description …”

“…Solutions using complex-valued approaches exist but are associated with increased cost. [17] Simply neglecting the continuum coupling altogether using CVS gives good results and is better than using a poor continuum description. [18] One important family of methods that has seen significant use to simulate X-ray spectroscopy is MC methods.…”

Coupled-cluster treatment of molecular strong-field ionization

“…25 CAPs have evolved to the most popular CV technique for autoionizing resonances (see Ref. 26 for an overview of recent work) and have also been used to investigate Stark resonances induced by time-dependent fields with explicitly time-dependent configurationinteraction singles (TD-CIS). [27][28][29] On the contrary, electronic-structure calculations in a basis of complex-scaled functions have been carried out only for autoionizing resonances.…”

Bound anionic states of DNA and RNA nucleobases: An EOM‐CCSD investigation

“…We keep the focus of this article limited to only bound anionic states. The study of quasibound or resonance states requires special techniques, the inclusion of which is outside the scope of the present study.…”