# Qubit Coupled Cluster Method: A Systematic Approach to Quantum Chemistry on a Quantum Computer

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posted on 14.11.2018, 00:00 by Ilya G. Ryabinkin, Tzu-Ching Yen, Scott N. Genin, Artur F. IzmaylovA unitary coupled cluster (UCC) form
for the wave function in the
variational quantum eigensolver has been suggested as a systematic
way to go beyond the mean-field approximation and include electron
correlation in solving quantum chemistry problems on a quantum computer.
Although being exact in the limit of including all possible coupled
cluster excitations, practically, the accuracy of this approach depends
on the number and type of terms are included in the wave function
parametrization. Another difficulty of UCC is a growth of the number
of simultaneously entangled qubits even at the

*fixed*Fermionic excitation rank. Not all quantum computing architectures can cope with this growth. To address both problems, we introduce a qubit coupled cluster (QCC) method that starts directly in the qubit space and uses energy response estimates for ranking the importance of individual entanglers for the variational energy minimization. Also, we provide an exact factorization of a unitary rotation of more than two qubits to a product of two-qubit unitary rotations. Thus, the QCC method with the factorization technique can be limited to only two-qubit entanglement gates and allows for very efficient use of quantum resources in terms of the number of coupled cluster operators. The method performance is illustrated by calculating ground-state potential energy curves of H_{2}and LiH molecules with chemical accuracy, ≤1 kcal/mol, and a symmetric water dissociation curve.## Read the peer-reviewed publication

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## Keywords

cluster excitationswave function parametrizationfactorization techniqueQuantum Chemistryrotationtermenergy response estimatesCluster MethodQuantum Computerquantum resourcesqubit spaceFermionic excitation rankH 2two-qubit entanglement gatesmethod performancemean-field approximationvariational energy minimizationcluster operatorschemical accuracySystematic Approachelectron correlationquantum chemistry problemsquantum computerUCCLiH moleculesenergy curvesvariational quantum eigensolverQCC methodwave functionwater dissociation curve

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## Read the peer-reviewed publication

## Categories

## Keywords

cluster excitationswave function parametrizationfactorization techniqueQuantum Chemistryrotationtermenergy response estimatesCluster MethodQuantum Computerquantum resourcesqubit spaceFermionic excitation rankH 2two-qubit entanglement gatesmethod performancemean-field approximationvariational energy minimizationcluster operatorschemical accuracySystematic Approachelectron correlationquantum chemistry problemsquantum computerUCCLiH moleculesenergy curvesvariational quantum eigensolverQCC methodwave functionwater dissociation curve