Quantum Simulation of Strongly Correlated Systems With a Generalized Unitary Coupled Cluster Method

The variational quantum eigensolver (VQE) is a hybrid algorithm that combines quantum and classical resources to effectively simulate energies of quantum systems. Within this framework, the preparation and propagation of the considered quantum state are achieved on the quantum processing unit (QPU), while the classical optimization routines for the parameters of the quantum state occur on the CPU. One of the biggest challenges in modern quantum chemistry involves the accurate modeling of multireference phenomena. A previous study applied the VQE to simulate molecular energies of strongly correlated systems using the unitary coupled cluster (UCC) ansatz. We introduce strategies for a further improvement through the implementation of a "generalized" UCC ansatz. While the "regular" UCC only considers occupied to virtual excitations, its generalized counterpart considers all possible excitations. In addition, matrix product states (MPS), which are inherently multireference, are considered for the initial states. Numerical simulations of several systems are presented, demonstrating advantages over the regular UCC ansatz and Hartree-Fock initial state.