Novel and predictive modeling tools for overcoming exponential computational barriers faced by  quantum chemistry methodologies are needed to describe chemical transformations that involve challenging quasi-degenerate electronic states that are commonly encountered in modeling chemical processes related to  catalysis, actinide chemistry, nitrogen fixation, and energy storage materials.  To address these challenges, we have recently introduced  an extension of  the sub-system embedding sub-algebras coupled cluster (SES-CC) formalism to the unitary CC  (UCC) formulation, which in contrast to the canonical single-reference SES-CC formalism,  results in a   Hermitian form of the  effective  Hamiltonian defined in the complete active spaces (CASs).  It was shown that the utilization of special parametrization of the UCC Ansatz, the so-called double unitary CC formalism (DUCC), provides a rigorous separation between  external (dynamical) and internal (static) correlation effects in the active space effective Hamiltonians. The proposed formalism can be viewed as an efficient way of downfolding many-electron Hamiltonian to the low-energy model represented by a particular choice of CAS. The Hermitian character of low-dimensionality effective Hamiltonians makes them an ideal target for quantum computing.  To illustrate the performance and feasibility of the DUCC formalism, we will provide preliminary results obtained with the Quantum Phase Estimator (QPE) for benchmark molecular systems described by Gaussian basis sets.

Coupled cluster downfolding techniques for quantum computing: Dimensionality reduction of electronic Hamiltonians based on active-space concepts

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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