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PreprintCC BYChemRxiv

Static correlation, a coupled-cluster diagnostic blind spot, and its quantum-computing resolution in silver–dinitrogen clusters

  • Mohsen Doust Mohammadi
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Abstract

The size-dependent adsorption of molecular nitrogen on small silver clusters, Ag n N 2 (n = 1–6), is investigated by density functional theory, and an adsorption energy of −0.44 eV, two-to seven fold stronger than for any neighboring cluster size, is identified for n = 3, accompanied by the only appreciable N–N bond elongation in the series. Complete active space self-consistent field calculations reveal that this coincides with static correlation that grows substantially along the N–N stretching coordinate, a correlation index rising from 1.14 to 4.15 between equilibrium and 2.40 Å. Over the same range, the Lee–Taylor T 1 diagnostic from coupled-cluster theory with single, double, and perturbative triple excitations is found to plateau between 0.03 and 0.05, and, for a second cluster, to fall from a maximum of 0.042 to 0.030 as the bond is stretched further. This diagnostic blind spot is confirmed, in a markedly amplified form, for the dissociation of free dinitrogen, where explicit wavefunction-stability analysis establishes that the coupled-cluster reference becomes variationally unstable precisely where the diagnostic offers no warning. Motivated by this finding, a sixteen-qubit unitary coupled-cluster variational quantum eigensolver calculation, comprising approximately 5,700 Pauli terms, is benchmarked against the multireference reference across the identical coordinate, reproducing the reference energy to within 0.29–0.39 mE h at equilibrium and 6.0 mEh at the most strongly correlated geometry examined. These results establish a concrete, cross-validated case study of a practically consequential coupled-cluster diagnostic limitation, together with a physically motivated demonstration that a variational quantum algorithm reproduces the correct multireference physics in a regime where the conventional single-reference benchmark cannot be trusted.

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Cite

BibTeXRIS
BibTeX
@misc{mohammadi2026static,
  title = {{Static correlation, a coupled-cluster diagnostic blind spot, and its quantum-computing resolution in silver–dinitrogen clusters}},
  author = {Mohsen Doust Mohammadi},
  howpublished = {ChemRxiv},
  year = {2026},
  month = aug,
  doi = {10.26434/chemrxiv.15008096/v1},
  url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008096/v1},
  note = {CC BY},
}
RIS
TY  - UNPB
TI  - Static correlation, a coupled-cluster diagnostic blind spot, and its quantum-computing resolution in silver–dinitrogen clusters
AU  - Mohsen Doust Mohammadi
PY  - 2026
DA  - 2026/08/31
PB  - ChemRxiv
DO  - 10.26434/chemrxiv.15008096/v1
UR  - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008096/v1
AB  - The size-dependent adsorption of molecular nitrogen on small silver clusters, Ag n N 2 (n = 1–6), is investigated by density functional theory, and an adsorption energy of −0.44 eV, two-to seven fold stronger than for any neighboring cluster size, is identified for n = 3, accompanied by the only appreciable N–N bond elongation in the series. Complete active space self-consistent field calculations reveal that this coincides with static correlation that grows substantially along the N–N stretching coordinate, a correlation index rising from 1.14 to 4.15 between equilibrium and 2.40 Å. Over the same range, the Lee–Taylor T 1 diagnostic from coupled-cluster theory with single, double, and perturbative triple excitations is found to plateau between 0.03 and 0.05, and, for a second cluster, to fall from a maximum of 0.042 to 0.030 as the bond is stretched further. This diagnostic blind spot is confirmed, in a markedly amplified form, for the dissociation of free dinitrogen, where explicit wavefunction-stability analysis establishes that the coupled-cluster reference becomes variationally unstable precisely where the diagnostic offers no warning. Motivated by this finding, a sixteen-qubit unitary coupled-cluster variational quantum eigensolver calculation, comprising approximately 5,700 Pauli terms, is benchmarked against the multireference reference across the identical coordinate, reproducing the reference energy to within 0.29–0.39 mE h at equilibrium and 6.0 mEh at the most strongly correlated geometry examined. These results establish a concrete, cross-validated case study of a practically consequential coupled-cluster diagnostic limitation, together with a physically motivated demonstration that a variational quantum algorithm reproduces the correct multireference physics in a regime where the conventional single-reference benchmark cannot be trusted.
ER  -