Assess finite-basis sensitivity in EOM-CC and EOM-UCC

Determine whether the anomalously large active-space EOM-CCSD error of approximately 8 eV, relative to comparable prior estimates, reflects a limitation of EOM-CC associated with finite-basis-set effects that is also present in EOM-UCC theory.

Background

The conclusion compares the reported q-sc-EOM-UCCSD results with prior work and notes a discrepancy in which an active-space EOM-CCSD calculation exhibits an approximately 8 eV error. The authors state that this error is unusually large compared with estimates reported elsewhere and that differences in active-space treatment, reference-state preparation, and possible fortuitous error cancellation complicate the comparison.

The unresolved question is whether sensitivity to finite-basis effects in EOM-CC provides evidence of a related limitation in EOM-UCC. Resolving this issue would help distinguish methodological deficiencies from discrepancies caused by basis choice, active-space truncation, or error cancellation.