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Title: Generalized Unitary Coupled Cluster Wave functions for Quantum Computation

Journal Article · · Journal of Chemical Theory and Computation

We introduce a unitary coupled-cluster (UCC) ansatz termed k-UpCCGSD that is based on a family of sparse generalized doubles operators, which provides an affordable and systematically improvable unitary coupled-cluster wave function suitable for implementation on a near-term quantum computer. k-UpCCGSD employs k products of the exponential of pair coupled-cluster double excitation operators (pCCD), together with generalized single excitation operators. We compare its performance in both efficiency of implementation and accuracy with that of the generalized UCC ansatz employing the full generalized single and double excitation operators (UCCGSD), as well as with the standard ansatz employing only single and double excitations (UCCSD). k-UpCCGSD is found to show the best scaling for quantum computing applications, requiring a circuit depth of O(kN), compared with O(N3) for UCCGSD, and O((N-η)2η) for UCCSD, where N is the number of spin orbitals and η is the number of electrons. We analyzed the accuracy of these three ansätze by making classical benchmark calculations on the ground state and the first excited state of H4 (STO-3G, 6-31G), H2O (STO-3G), and N2 (STO-3G), making additional comparisons to conventional coupled cluster methods. The results for ground states show that k-UpCCGSD offers a good trade-off between accuracy and cost, achieving chemical accuracy for lower cost of implementation on quantum computers than both UCCGSD and UCCSD. UCCGSD is also found to be more accurate than UCCSD but at a greater cost for implementation. Excited states are calculated with an orthogonally constrained variational quantum eigensolver approach. This is seen to generally yield less accurate energies than for the corresponding ground states. Lastly, we demonstrate that using a specialized multideterminantal reference state constructed from classical linear response calculations allows these excited state energetics to be improved.

Research Organization:
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES); USDOE Office of Science (SC), Advanced Scientific Computing Research (ASCR)
Grant/Contract Number:
AC02-05CH11231
OSTI ID:
1605226
Journal Information:
Journal of Chemical Theory and Computation, Vol. 15, Issue 1; ISSN 1549-9618
Publisher:
American Chemical SocietyCopyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 147 works
Citation information provided by
Web of Science

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Cited By (12)

Downfolding of many-body Hamiltonians using active-space models: Extension of the sub-system embedding sub-algebras approach to unitary coupled cluster formalisms journal July 2019
An adaptive variational algorithm for exact molecular simulations on a quantum computer journal July 2019
Variational quantum eigensolver with fewer qubits journal September 2019
Exact parameterization of fermionic wave functions via unitary coupled cluster theory journal December 2019
An adaptive variational algorithm for exact molecular simulations on a quantum computer text January 2018
Variational Quantum Computation of Excited States journal July 2019
Variational Quantum Eigensolver with Fewer Qubits text January 2019
A general time-domain formulation of equation-of-motion coupled-cluster theory for linear spectroscopy journal November 2019
Variational Quantum Computation of Excited States text January 2018
Quantum computational chemistry text January 2018
Exact Parameterization of Fermionic Wave Functions via Unitary Coupled Cluster Theory text January 2019
Quantum Computation of Electronic Transitions using a Variational Quantum Eigensolver text January 2019

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