Rapidly convergent quantum Monte Carlo using a Chebyshev projector.

The multireference coupled-cluster Monte Carlo (MR-CCMC) algorithm is a determinant-based quantum Monte Carlo (QMC) algorithm that is conceptually similar to Full Configuration Interaction QMC (FCIQMC). It has been shown to offer a balanced treatment of both static and dynamic correlation while retaining polynomial scaling, although application to large systems with significant strong correlation remained impractical. In this paper, we document recent algorithmic advances that enable rapid convergence and a more black-box approach to the multireference problem.

Folded Spectrum VQE: A Quantum Computing Method for the Calculation of Molecular Excited States.

The recent developments of quantum computing present novel potential pathways for quantum chemistry as the scaling of the computational power of quantum computers could be harnessed to naturally encode and solve electronic structure problems. Theoretically exact quantum algorithms for chemistry have been proposed (e.g.

Corrected density functional theory and the random phase approximation: Improved accuracy at little extra cost.

We recently introduced an efficient methodology to perform density-corrected Hartree-Fock density functional theory [DC(HF)-DFT] calculations and an extension to it we called "corrected" HF DFT [C(HF)-DFT] [Graf and Thom, J. Chem. Theory Comput.

Studies on the Transcorrelated Method.

We investigate the possibility of using a transcorrelated (TC) Hamiltonian to describe electron correlation. A method to obtain TC wavefunctions was developed based on the mathematical framework of the bi-variational principle. This involves the construction of an effective TC Hamiltonian matrix, which can be solved in a self-consistent manner.

Simple and Efficient Route toward Improved Energetics within the Framework of Density-Corrected Density Functional Theory.

The crucial step in density-corrected Hartree-Fock density functional theory (DC(HF)-DFT) is to decide whether the density produced by the density functional for a specific calculation is erroneous and, hence, should be replaced by, in this case, the HF density. We introduce an indicator, based on the difference in noninteracting kinetic energies between DFT and HF calculations, to determine when the HF density is the better option. Our kinetic energy indicator directly compares the self-consistent density of the analyzed functional with the HF density, is size-intensive, reliable, and most importantly highly efficient.

A hybrid stochastic configuration interaction-coupled cluster approach for multireference systems.

The development of multireference coupled cluster (MRCC) techniques has remained an open area of study in electronic structure theory for decades due to the inherent complexity of expressing a multiconfigurational wavefunction in the fundamentally single-reference coupled cluster framework. The recently developed multireference-coupled cluster Monte Carlo (mrCCMC) technique uses the formal simplicity of the Monte Carlo approach to Hilbert space quantum chemistry to avoid some of the complexities of conventional MRCC, but there is room for improvement in terms of accuracy and, particularly, computational cost. In this paper, we explore the potential of incorporating ideas from conventional MRCC-namely, the treatment of the strongly correlated space in a configuration interaction formalism-to the mrCCMC framework, leading to a series of methods with increasing relaxation of the reference space in the presence of external amplitudes.

Trade-Off between Redox Potential and the Strength of Electrochemical CO Capture in Quinones.

Electrochemical carbon dioxide capture recently emerged as a promising alternative approach to conventional energy-intensive carbon-capture methods. A common electrochemical capture approach is to employ redox-active molecules such as quinones. Upon electrochemical reduction, quinones become activated for the capture of CO through a chemical reaction.

Making the most of data: Quantum Monte Carlo postanalysis revisited.

In quantum Monte Carlo (QMC) methods, energy estimators are calculated as (functions of) statistical averages of quantities sampled during a calculation. Associated statistical errors of these averages are often estimated. This error estimation is not straightforward and there are several choices of the error estimation methods.

Localized Spin Rotations: A Size-Consistent Approach to Nonorthogonal Configuration Interaction.

Current nonorthogonal configuration interaction (NOCI) methods often use a set of self-consistent field (SCF) states selected based on chemical intuition. However, it may be challenging to track these SCF states across a dissociation profile and the NOCI states recovered may be spin contaminated. In this Article, we propose a method of applying spin rotation on symmetry broken unrestricted Hartree-Fock (sb-UHF) states to generate a basis for NOCI.

Software for the frontiers of quantum chemistry: An overview of developments in the Q-Chem 5 package.

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods.

A stochastic approach to unitary coupled cluster.

Unitary coupled cluster (UCC), originally developed as a variational alternative to the popular traditional coupled cluster method, has seen a resurgence as a functional form for use on quantum computers. However, the number of excitors present in the Ansatz often presents a barrier to implementation on quantum computers. Given the natural sparsity of wavefunctions obtained from quantum Monte Carlo methods, we consider here a stochastic solution to the UCC problem.

Towards a Holomorphic Density Functional Theory.

Self-consistent-field (SCF) approximations formulated using Hartree-Fock (HF) or Kohn-Sham density-functional theory (KS-DFT) have the potential to yield multiple solutions. However, the formal relationship between multiple solutions identified using HF or KS-DFT remains generally unknown. We investigate the connection between multiple SCF solutions for HF or KS-DFT by introducing a parameterized functional that scales between the two representations.

Theory and implementation of a novel stochastic approach to coupled cluster.

We present a detailed discussion of our novel diagrammatic coupled cluster Monte Carlo (diagCCMC) [Scott et al. J. Phys.

Reaching Full Correlation through Nonorthogonal Configuration Interaction: A Second-Order Perturbative Approach.

Nonorthogonal multireference methods can predict statically correlated adiabatic energies while providing chemical insight through the combination of diabatic reference states. However, reaching quantitative accuracy using nonorthogonal multireference expansions remains a significant challenge. In this work, we present the first rigorous perturbative correction to nonorthogonal configuration interaction, allowing the remaining dynamic correlation to be reliably computed.

Periodicity of Single-Molecule Magnet Behaviour of Heterotetranuclear Lanthanide Complexes across the Lanthanide Series: A Compendium.

Acetato-bridged palladium-lanthanide tetranuclear heterometallic complexes of the form [Pd Ln (H O) (CH COO) ]⋅2 CH COOH [Ln =Ce (1), Pr (2), Nd (3), Sm (4), Tb (5), Dy (6), Dy Y (6''), Ho (7), Er (8), Er Y (8''), Tm (9), Yb (10), Y (11)] were synthesised and characterised by experimental and theoretical techniques. All complexes containing Kramers lanthanide ions [Ln =Ce (1), Nd (3), Sm (4), Dy (6), DyY (6''), Er (8), ErY (8''), Yb (10)] showed field-induced slow magnetic relaxation, characteristic of single-molecule magnetism and purely of molecular origin. In contrast, all non-Kramers lanthanide ions [Ln =Pr (2), Tb (5), Ho (7), Tm (9), Y (11) is diamagnetic and non-lanthanide] did not show any slow magnetic relaxation.

Accelerating Convergence in Fock Space Quantum Monte Carlo Methods.

The convergence of full configuration interaction quantum Monte Carlo (FCIQMC) is accelerated using a quasi-Newton propagation (QN) which can also be applied to coupled cluster Monte Carlo (CCMC). The computational scaling of this optimized propagation is [Formula: see text], keeping the additional computational cost to a bare minimum. Its effects are investigated deterministically and stochastically on a model system, the uniform electron gas, with Hilbert space size up to 10, and shown to accelerate convergence of the instantaneous projected energy by over an order of magnitude in the FCIQMC test case.

An Organic-Inorganic Hybrid Exhibiting Electrical Conduction and Single-Ion Magnetism.

The first three-dimensional (3D) conductive single-ion magnet (SIM), (TTF) [Co(pdms) ] (TTF=tetrathiafulvalene and H pdms=1,2-bis(methanesulfonamido)benzene), was electrochemically synthesised and investigated structurally, physically, and theoretically. The similar oxidation potentials of neutral TTF and the molecular precursor [HNEt ] [M(pdms) ] (M=Co, Zn) allow for multiple charge transfers (CTs) between the SIM donor [M(pdms) ] and the TTF acceptor, as well as an intradonor CT from the pdms ligand to Co ion upon electrocrystallisation. Usually TTF functions as a donor, whereas in our system TTF is both a donor and an accepter because of the similar oxidation potentials.