Quantum chemistry simulation of ground- and excited-state properties of the sulfonium cation on a superconducting quantum processor

Abstract

The computational description of correlated electronic structure, and particularly of excited states of many-electron systems, is an anticipated application for quantum devices. An important ramification is to determine the dominant molecular fragmentation pathways in...

Quantum computation of π → π* and n → π* excited states of aromatic heterocycles

“…In this work, we performed ground-state calculations as a starting point for the subsequent determination of excited states. For each active space (see "Calculation Details" for their construction), we approximated the ground-state wavefunction using the entanglement forging (EF) technique [31,32]. One of the main challenges for near-term quantum algorithms, and the motivation behind EF, lies in the resources required for mapping molecular operators and wavefunctions onto qubits.…”

“…To model excited states, we used a quantum subspace expansion (QSE) built upon the EF ground state [26,27,31]. Within the QSE, the time-independent Schrödinger equation for ground and excited states is projected into a subspace of the manyelectron Hilbert space, spanned by a basis of wavefunctions of the form…”

“…Recent years have witnessed the development of quantum computing algorithms [18][19][20][21][22] for studying ground and excited states of electronic systems. These include variational methods [23][24][25] and subspace methods like the quantum subspace expansion (QSE) [26][27][28] and the quantum equation-of-motion [29], which build upon a groundstate approximation to calculate excited-state energies and properties [25,27,30,31]. While promising and rapidly progressing, these approaches have a number of challenges: (i) like classical algorithms, quantum computing algorithms are based on approximation schemes.…”

“…With these challenges in mind, we investigated the low-lying valence excited states of four common aromatic molecules, namely furan, pyrrole, pyridine, and pyrimidine. We constructed an active space of π → π * and n → π * valence transitions in which we approximated the ground-state wavefunction with a variational quantum computing algorithm based on a qubit reduction technique, called entanglement forging (EF) [31,32]. We then approximated the excited states with a QSE based on single and double electronic excitations applied to the ground-state wavefunction yielded by EF [26,27].…”

Statistical errors in reduced density matrices sampled from quantum circuit simulation and the impact on multireference perturbation theory

“…As will be mentioned in detail in the subsequent section, the electronic energies of target molecules are calculated by sampling of reduced density matrices (RDMs) for the electrons in the active space orbitals from quantum circuits and subsequently parsing the RDMs to classical computers, [30][31][32][33] which is often called a quantum-classical hybrid algorithm. For the CAS wavefunctions represented on qubits, these active-space RDMs are obtained by repeatedly observing the superposition states on the qubits and statistical estimation; if an infinite number of observations is performed, an exact RDM will be obtained.…”