A quantum computing view on unitary coupled cluster theory.

We present a review of the Unitary Coupled Cluster (UCC) ansatz and related ansätze which are used to variationally solve the electronic structure problem on quantum computers. A brief history of coupled cluster (CC) methods is provided, followed by a broad discussion of the formulation of CC theory. This includes touching on the merits and difficulties of the method and several variants, UCC among them, in the classical context, to motivate their applications on quantum computers.

On the order problem in construction of unitary operators for the variational quantum eigensolver.

One of the main challenges in the variational quantum eigensolver (VQE) framework is construction of the unitary transformation. The dimensionality of the space for unitary rotations of N qubits is 4- 1, which makes the choice of a polynomial subset of generators an exponentially difficult process. Moreover, due to non-commutativity of generators, the order in which they are used strongly affects results.

Excess electrons bound to HS trimer and tetramer clusters.

We have prepared the hydrogen sulfide trimer and tetramer anions, (HS) and (HS), measured their anion photoelectron spectra, and applied high-level quantum chemical calculations to interpret the results. The sharp peaks at low electron binding energies in their photoelectron spectra and their diffuse Dyson orbitals are evidence for them both being dipole-bound anions. While the dipole moments of the neutral (HS) and (HS) clusters are small, the excess electron induces structural distortions that enhance the charge-dipolar attraction and facilitate the binding of diffuse electrons.

Dyson Orbitals and Double Rydberg Anions: Methylated, Annulated, and Paramagnetic.

A double Rydberg anion (DRA) consists of a saturated, closed-shell, molecular cation and two electrons that occupy diffuse orbitals. Techniques of electron propagator theory (EPT) predict the existence and spectra of three new classes of DRAs. The first, with the formula NH(CH), has vertical electron detachment energies (VEDEs) that vary between 0.

Electron Propagator Methods for Vertical Electron Detachment Energies of Anions: Benchmarks and Case Studies.

Ab initio electron propagator methods are efficient and accurate means of calculating vertical electron detachment energies of closed-shell, molecular anions with nuclei from the first three periods. Basis set extrapolations enable definitive comparisons between electron propagator results and benchmarks defined by total energy differences obtained with coupled-cluster, single, double, plus perturbative triple substitution theory. The best compromises of accuracy and efficiency are provided by the renormalized, partial third-order, diagonal (P3+) self-energy and by the nondiagonal, renormalized, second-order (NR2) approximation.

Comment on "Are polynuclear superhalogens without halogen atoms probable? A high-level ab initio case study on triple-bridged binuclear anions with cyanide ligands" [J. Chem. Phys. 140, 094301 (2014)].

For the vertical electron detachment energies of triply-bridged Mg(CN) superhalides, the Outer Valence Green Function (OVGF) yields results similar to those of the coupled-cluster singles and doubles plus approximate triples, or CCSD(T), method. Invalid comparisons between states with different symmetry or localization properties underlie the assertion of Yin et al that OVGF produces large discrepancies with respect to CCSD(T) for several isomers of Mg(CN).

Composite electron propagator methods for calculating ionization energies.

Accurate ionization energies of molecules may be determined efficiently with composite electron-propagator (CEP) techniques. These methods estimate the results of a calculation with an advanced correlation method and a large basis set by performing a series of more tractable calculations in which large basis sets are used with simpler approximations and small basis sets are paired with more demanding correlation techniques. The performance of several CEP methods, in which diagonal, second-order electron propagator results with large basis sets are combined with higher-order results obtained with smaller basis sets, has been tested for the ionization energies of closed-shell molecules from the G2 set.

Comment on "Is the regulation of the electronic properties of organic molecules by polynuclear superhalogens more effective than that by mononuclear superhalogens? A high-level ab initio case study" by M.-M. Li, J.-F. Li, H.-C. Bai, Y.-Y. Sun, J.-L. Li and B. Yin, Phys. Chem. Chem. Phys., 2015, 17, 20338.

The Outer Valence Green Function (OVGF) and coupled-cluster singles and doubles plus approximate triples, or CCSD(T), methods yield similar results for the vertical detachment energies of superhalides studied recently by Li et al. The success of the OVGF method contradicts claims by Li et al. in their recent article.

Accurate Ionization Potentials and Electron Affinities of Acceptor Molecules IV: Electron-Propagator Methods.

Comparison of ab initio electron-propagator predictions of vertical ionization potentials and electron affinities of organic, acceptor molecules with benchmark calculations based on the basis set-extrapolated, coupled cluster single, double, and perturbative triple substitution method has enabled identification of self-energy approximations with mean, unsigned errors between 0.1 and 0.2 eV.