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The Coupled Cluster (CC) method is used to compute the electronic correlationenergy in atoms and molecules and often leads to highly accurate results.However, due to its single-reference nature, standard CC in its projected formfails to describe quantum states characterized by strong electroniccorrelations and multi-reference projective methods become necessary. On theother hand, quantum algorithms for the solution of many-electron problems havealso emerged recently. The quantum UCC with singles and doubles (q-UCCSD) is apopular wavefunction Ansatz for the Variational Quantum Eigensolver (VQE)algorithm. The variational nature of this approach can lead to significantadvantages compared to its classical equivalent in the projected form, inparticular for the description of strong electronic correlation. However, dueto the large number of gate operations required in q-UCCSD, approximations needto be introduced in order to make this approach implementable in astate-of-the-art quantum computer. In this work, we evaluate several variantsof the standard q-UCCSD Ansatz in which only a subset of excitations isincluded. In particular, we investigate the singlet and pair q-UCCD approachescombined with orbital optimization. We show that these approaches can capturethe dissociation/distortion profiles of challenging systems such as H$_4$,H$_2$O and N$_2$ molecules, as well as the one-dimensional periodicFermi-Hubbard chain. These results promote the future use of q-UCC methods forthe solution of challenging electronic structure problems in quantum chemistry.

journal of chemical physics online/the journal of chemical physics/journal of chemical physics

Quantum Orbital-Optimized Unitary Coupled Cluster Methods in the  Strongly Correlated Regime: Can Quantum Algorithms Outperform their Classical  Equivalents?