The variational quantum eigensolver (VQE) algorithm, designed to calculate the energy of molecular ground states on near‐term quantum computers, requires specification of symmetries that describe the system, for example, spin state and number of electrons. This opens the possibility of using VQE to obtain excited states as the lowest‐energy solutions of a given set of symmetries. In this paper, the performances of various unitary coupled cluster (UCC) ansätze applied to VQE calculations on excited states are investigated using quantum circuits designed to represent single reference and multireference wavefunctions to calculate energy curves with respect to variations in the molecular geometry. These ansätze include standard Unitary Coupled Cluster Singles and Doubles (UCCSD), as well as modified versions of Unitary Coupled Cluster Generalized Singles and Doubles (UCCGSD) and k‐UpCCGSD, which are engineered to tackle excited states without undesired spin symmetry crossover to lower states during VQE optimization. These studies are carried out on a range of systems, including H2, H3, H4, NH, OH+, CH2, and , covering examples of spin singlet, doublet, and triplet molecular ground states with single and multireference excited states. In most cases, our calculations are in excellent agreement with results from full‐configuration interaction calculations on classical machines, thus showing that the VQE algorithm is capable of calculating the lowest excited state at a certain symmetry, including multireference closed‐ and open‐shell states, by setting appropriate restrictions on the excitations considered in the cluster operator and appropriate constraints in the qubit register encoding the starting mean field state.

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We have demonstrated a prototypical hybrid classical and quantum computational workflow for the quantification of protein-ligand interactions. The workflow combines the density matrix embedding theory (DMET) embedding procedure with the variational quantum eigensolver (VQE) approach for finding molecular electronic ground states. A series of β -secretase (BACE1) inhibitors is rank-ordered using binding energy differences calculated on the latest superconducting transmon (IBM) and trapped-ion (Quantinuum) noisy intermediate scale quantum (NISQ) devices. This is the first application of real quantum computers to the calculation of protein-ligand binding energies. The results shed light on hardware and software requirements which would enable the application of NISQ algorithms in drug design. | INTRODUCTIONThe advent of quantum mechanics at the turn of the 20th century changed the way we look at the physical sciences. For chemistry, the implications were profound, as Dirac famously noted: "the fundamental laws … for the whole of chemistry are thus completely known, and the difficulty lies only in the fact that application of these laws leads to equations that are too complex to be solved" [1]. Indeed, calculations of accurate solutions of the electronic Schrödinger equation, such as the full configuration interaction method (FCI), of molecules scale exponentially with the number of atoms [2], rendering them applicable only to the smallest systems. Practical approximations to FCI, such as, for example, CCSD(T) [3], touted as computational chemistry's gold standard, do exist, but their applicability is limited: single-reference methods like CCSD(T) fail for strongly correlated ("multireference") systems and the formal scaling of CCSD(T) with system size is O(N 7 ), rendering it useful only for relatively small molecules. While the scaling limitation of CCSD(T) can be dramatically reduced via localized methods such as DLPNO [4] at the expense of accuracy, FCI-based methods for strongly correlated systems scale exponentially with the number of correlated electrons and, even for systems small enough to be tractable, require expert knowledge to be applied. Emerging approaches for strongly correlated systems, such as DMRG [5] and selected CI [6] can achieve results close to FCI at lower cost, thereby extending the limit to about 100 orbitals. In most cases, the correlated method is applied to an active space of selected orbitals, the choice of which has traditionally been arbitrary but can also be performed by automated procedures [7,8]. On the other hand, mean-field methods, such as Hartree-Fock (HF) and density functional theory (DFT), which either dispense with electron correlation completely or treat it in an approximate, implicit manner, are routinely applied to organic and inorganic systems of sizes up to few hundred atoms. This is possible due to their relatively low, O N 3 -O N 4 À Á , formal scaling, which could be reduced even to O N ð Þ in approximate implementations for systems comprised of thousands of atoms...

A joint experimental and theoretical framework for the decoupling of boron (B) doping and stoichiometric-induced modifications to the structural properties and electronic band structure of germanium (Ge)/AlAs(001) heterostructures is presented. The effect of B-induced stress on nearest-neighbor Ge bonds is quantified via X-ray diffractometry and Raman spectroscopic analysis and subsequently interpreted through the lens of density functional perturbation theory. Similarly, experimental determination of the energy band alignment at the p-type Ge:B/AlAs heterointerface is understood using a density functional theory approach to model the influence of heterointerface stoichiometry and interatomic bonding between group IV and III–V interfacial atoms on the valence and conduction band discontinuities. The modeled two monolayer interatomic diffusion at the Ge/AlAs heterointerface is confirmed via atom probe tomography analysis, demonstrating a ∼6 Å interfacial width. These results present a unified picture of the Ge:B/AlAs(001) material system, highlighting the influence of B on its structural and electronic properties, and provide a path for the engineering of such heterointerfaces through high concentration dopant incorporation within the overlying Ge epilayer.

First-principles calculations are applied to study the formation energies of various divacancy defects in armchair and zigzag carbon nanotubes of varying diameter, and the transport properties for the corresponding structures. Our explicit ab initio calculations confirm that the lateral 585 divacancy is the most stable defect in small diameter tubes, with the 555 777 divacancy becoming more stable in armchair tubes larger than (30, 30). Evaluating the electron transmission as a function of diameter and chirality for a range of defects, the strongest scattering is found for the 555 777 divacancy configuration, which is observable in electrical spectroscopy experiments. Finally, validation of an approximation relating contributions from independent scattering sites enables the study of the characteristic localization length in large diameter tubes. Despite the fixed number of channels, localization lengths increase with increasing diameter and can exceed 100 nm for typical defect densities.