# Evaluating Ground State Energies of Chemical Systems with Low-Depth Quantum Circuits and High Accuracy

## TL;DR

This paper introduces a new quantum computing method that reduces the number of parameters needed for accurate chemical simulations.

## Contribution

The novel contribution is an enhanced QCC ansatz for VQE that reduces parameters while maintaining accuracy.

## Key findings

- The enhanced QCC ansatz requires only n parameters instead of n + 2m.
- The method achieves high accuracy for ground state energies of strongly correlated molecules.
- Experiments on real quantum hardware demonstrate the practicality of the approach.

## Abstract

Quantum computers
have the potential to efficiently solve
the electronic
structure problem but are currently limited by noise and shallow circuits.
We present an enhanced Variational Quantum Eigensolver (VQE) ansatz
based on the Qubit Coupled Cluster (QCC) approach that requires optimization
of only n parameters, where n is
the number of Pauli string generators, rather than the typical n + 2m parameters, where m is the number of qubits. We evaluate the ground state energies and
molecular dissociation curves of strongly correlated molecules, namely
O3 and Li4, using active spaces of varying sizes
in conjunction with our enhanced QCC ansatz, Unitary Coupled Cluster
Single–Double (UCCSD) ansatz, and the classical Coupled Cluster
Singles and Doubles (CCSD) method. Compared to UCCSD, our approach
significantly reduces the number of parameters while maintaining high
accuracy. Numerical simulations demonstrate the effectiveness of our
approach, and experiments on superconducting and trapped-ion quantum
computers showcase its practicality on real hardware. By eliminating
the need for symmetry-restoring gates and reducing the number of parameters,
our enhanced QCC ansatz enables accurate quantum chemistry calculations
on near-term quantum devices for strongly correlated systems.