Journal
MOLECULAR PHYSICS
Volume 119, Issue 21-22, Pages -Publisher
TAYLOR & FRANCIS LTD
DOI: 10.1080/00268976.2021.1943029
Keywords
Quantum embedding; coupled-cluster theory; equation-of-motion coupled-cluster theory; excited states; electronic resonances
Funding
- European Research Council (ERC) under the European Union [851766]
- Research Foundation -Flanders (FWO)
- Flemish Government
- European Research Council (ERC) [851766] Funding Source: European Research Council (ERC)
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The projection-based quantum embedding method is successfully applied to various electronic states, showing excellent performance in describing ionization and valence excitation transitions. However, more caution is needed when dealing with Rydberg excitations or electron attachment, requiring the use of long-range corrected density functionals and facing difficulties with virtual orbital space truncation.
The projection-based quantum embedding method is applied to a comprehensive set of electronic states. We embed different variants of equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) theory in density functional theory and investigate electronically excited states of valence, Rydberg, and charge-transfer character, valence- and core-ionised states, as well as bound and temporary radical anions. The latter states, which are unstable towards electron loss, are treated by means of a complex-absorbing potential. Besides transition energies, we present Dyson orbitals and natural transition orbitals for embedded EOM-CCSD. We benchmark the performance of the embedded EOM-CCSD methods against full EOM-CCSD using small organic molecules microsolvated by a varying number of water molecules as test cases. Our results illustrate that embedded EOM-CCSD describes ionisation and valence excitation very well and that these transitions are quite insensitive towards technical details of the embedding procedure. On the contrary, more care is required when dealing with Rydberg excitations or electron attachment. For the latter type of transition in particular, the use of long-range corrected density functionals is mandatory and truncation of the virtual orbital space proves to be difficult.
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