Exploring Electron Affinities, LUMO Energies, and Band Gaps with Electron-Pair Theories.

We introduce the electron attachment equation-of-motion pair coupled cluster doubles (EA-EOM-pCCD) ansatz, which allows us to inexpensively compute electron affinities, energies of unoccupied orbitals, and electron attachment spectra. We assess the accuracy of EA-EOM-pCCD for a representative data set of organic molecules for which experimental data are available, as well as the electron attachment process in uranyl dichloride. EA-EOM-pCCD provides more reliable energies for electron attachment properties than its ionization potential EOM counterpart.

Linear Response pCCD-Based Methods: LR-pCCD and LR-pCCD+S Approaches for the Efficient and Reliable Modeling of Excited State Properties.

In this work, we derive working equations for the linear response pair coupled cluster doubles (LR-pCCD) ansatz and its extension to singles (S), LR-pCCD+S. These methods allow us to compute electronic excitation energies and transition dipole moments based on a pCCD reference function. We benchmark the LR-pCCD+S model against the linear response coupled-cluster singles and doubles method for modeling electronic spectra (excitation energies and transition dipole moments) of the BH, HO, HCO, and furan molecules.

Towards reliable and efficient modeling of [CuO]-based compound electronic structures with the partially fixed reference space protocols.

This work reports a computationally efficient approach for reliable modeling of complex electronic structures based on [CuO] moieties. Specifically, we explore the recently developed partially fixed reference space (PFRS) protocol to minimize the active space size, taking into account the double d-shell effects. We show that the ground-state electronic structure of the core [CuO] model system is dominated by the d/d occupations.

Delving into the catalytic mechanism of molybdenum cofactors: a novel coupled cluster study.

In this work, we use modern electronic structure methods to model the catalytic mechanism of different variants of the molybdenum cofactor (Moco). We investigate the dependence of various Moco model systems on structural relaxation and the importance of environmental effects for five critical points along the reaction coordinate with the DMSO and NO substrates. Furthermore, we scrutinize the performance of various coupled-cluster approaches for modeling the relative energies along the investigated reaction paths, focusing on several pair coupled cluster doubles (pCCD) flavors and conventional coupled cluster approximations.

Toward Reliable Dipole Moments without Single Excitations: The Role of Orbital Rotations and Dynamical Correlation.

The dipole moment is a crucial molecular property linked to a molecular system's bond polarity and overall electronic structure. To that end, the electronic dipole moment, which results from the electron density of a system, is often used to assess the accuracy and reliability of new electronic structure methods. This work analyses electronic dipole moments computed with the pair coupled cluster doubles (pCCD) ansätze and its linearized coupled cluster (pCCD-LCC) corrections using the canonical Hartree-Fock and pCCD-optimized (localized) orbital bases.

Accelerating Pythonic Coupled-Cluster Implementations: A Comparison Between CPUs and GPUs.

In this work, we benchmark several Python routines for time and memory requirements to identify the optimal choice of the tensor contraction operations available. We scrutinize how to accelerate the bottleneck tensor operations of Pythonic coupled-cluster implementations in the Cholesky linear algebra domain, utilizing a NVIDIA Tesla V100S PCIe 32GB (rev 1a) graphics processing unit (GPU). The NVIDIA compute unified device architecture API interacts with CuPy, an open-source library for Python, designed as a NumPy drop-in replacement for GPUs.

Geminal-Based Strategies for Modeling Large Building Blocks of Organic Electronic Materials.

We elaborate on unconventional electronic structure methods based on geminals and their potential to advance the rapidly developing field of organic photovoltaics (OPVs). Specifically, we focus on the computational advantages of geminal-based methods over standard approaches and identify the critical aspects of OPV development. Examples are reliable and efficient computations of orbital energies, electronic spectra, and van der Waals interactions.

The relationship between structure and excited-state properties in polyanilines from geminal-based methods.

We employ state-of-the-art quantum chemistry methods to study the structure-to-property relationship in polyanilines (PANIs) of different lengths and oxidation states. Specifically, we focus on leucoemeraldine, emeraldine, and pernigraniline in their tetramer and octamer forms. We scrutinize their structural properties, HOMO and LUMO energies, HOMO-LUMO gaps, and vibrational and electronic spectroscopy using various Density Functional Approximations (DFAs).

Static embedding with pair coupled cluster doubles based methods.

Quantum embedding methods have recently been significantly developed to model large molecular structures. This work proposes a novel wave function theory in a density functional theory (WTF-in-DFT) embedding scheme based on pair-coupled cluster doubles (pCCD)-type methods. While pCCD can reliably describe strongly-correlated systems with mean-field-like computational cost, the large extent of the dynamic correlation can be accounted for by (linearized) coupled-cluster corrections on top of the pCCD wave function.

Diffuse Basis Functions for Relativistic s and d Block Gaussian Basis Sets.

Diffuse s, p, and d functions have been optimized for use with previously reported relativistic basis sets for the s and d blocks of the periodic table. The functions were optimized on the 4:1 weighted average of the s and p configurations of the anion, with the d shell in the d configuration for the d blocks. Exponents were extrapolated for groups 2 and 12, which have unstable or weakly bound anions.

Geminal-based electronic structure methods in quantum chemistry. Toward a geminal model chemistry.

In this review, we discuss the recent progress in developing geminal-based theories for challenging problems in quantum chemistry. Specifically, we focus on the antisymmetrized geminal power, generalized valence bond, antisymmetrized product of strongly orthogonal geminals, singlet-type orthogonal geminals, the antisymmetric product of 1-reference orbital geminal, also known as the pair coupled cluster doubles ansatz, and geminals constructed from Richardson-Gaudin states. Furthermore, we review various corrections to account for the missing dynamical correlation effects in geminal models and possible extensions to target electronically excited states and open-shell species.

Resolving the π-assisted U-N σ-bond formation using quantum information theory.

We model the potential energy profiles of the UO (NCO)Cl → NUOCl + CO reaction pathway [Y. Gong, V. Vallet, M.

Multi-reference ab initio calculations of Hg spectral data and analysis of magic and zero-magic wavelengths.

We have identified magic wavelengths for S ↔ P (m = 0) transitions and zero-magic wavelengths for the P (m = 0) states of Hg atoms, analysed the robustness of the magic conditions with respect to wavelength and polarization imperfections. We show that the most experimentally feasible magic wavelength for the S ↔ P transition is 351.8 nm of π polarized light.

Mixed uranyl and neptunyl cation-cation interaction-driven clusters: structures, energetic stability, and nuclear quadrupole interactions.

We present a state-of-the-art quantum chemical study of mixed cation-cation interaction (CCI) driven complexes composed of uranyl and neptunyl units. Specifically, we consider the stability of the D-shaped and T-shaped structural rearrangements in CCIs, various oxidation states of the uranium and neptunium atom (v and vi), as well as a different number of unpaired electrons. Furthermore, we scrutinize the nuclear quadrupole interactions of the bare actinyl subunits and the most stable mixed CCI clusters.

Assessing the accuracy of simplified coupled cluster methods for electronic excited states in f0 actinide compounds.

We scrutinize the performance of different variants of equation of motion coupled cluster (EOM-CC) methods to predict electronic excitation energies and excited state potential energy surfaces in closed-shell actinide species. We focus our analysis on various recently presented pair coupled cluster doubles (pCCD) models [J. Chem.

Benchmarking the Accuracy of Seniority-Zero Wave Function Methods for Noncovalent Interactions.

In this paper, we scrutinize the ability of seniority-zero wave function-based methods to model different types of noncovalent interactions, such as hydrogen bonds, dispersion, and mixed noncovalent interactions as well as prototypical model systems with various contributions of dynamic and static electron correlation effects. Specifically, we focus on the pair Coupled Cluster Doubles (pCCD) ansatz combined with two different flavors of dynamic energy corrections, (i) based on a perturbation theory correction and (ii) on a linearized coupled cluster ansatz on top of pCCD. We benchmark these approaches against the A24 data set [ Řezáč and Hobza J.

Elucidating cation-cation interactions in neptunyl dications using multi-reference ab initio theory.

Understanding the binding mechanism in neptunyl clusters formed due to cation-cation interactions is of crucial importance in nuclear waste reprocessing and related areas of research. Since experimental manipulations with such species are often rather limited, we have to rely on quantum-chemical predictions of their electronic structures and spectroscopic parameters. In this work, we present a state-of-the-art quantum chemical study of the T-shaped and diamond-shaped neptunyl(v) and neptunyl(vi) dimers.

Electron correlation effects of the ThO and ThS molecules in the spinor basis. A relativistic coupled cluster study of ground and excited states properties.

We present a comprehensive relativistic coupled cluster study of the electronic structures of the ThO and ThS molecules in the spinor basis. Specifically, we use the single-reference coupled cluster and the multi-reference Fock Space Coupled Cluster (FSCC) methods to model their ground and electronically-excited states. Two variants of the FSCC method have been investigated: (a) one where the electronic spectrum is obtained from sector (1,1) of the Fock space, and (b) another where the excited states come from the doubly attached electronic states to the doubly charged systems (ThO2+ and ThS2+), that is, from sector (0,2) of the Fock space.

Benchmark of Dynamic Electron Correlation Models for Seniority-Zero Wave Functions and Their Application to Thermochemistry.

Wave functions restricted to electron-pair states are promising models to describe static/nondynamic electron correlation effects encountered, for instance, in bond-dissociation processes and transition-metal and actinide chemistry. To reach spectroscopic accuracy, however, the missing dynamic electron correlation effects that cannot be described by electron-pair states need to be included a posteriori. In this Article, we extend the previously presented perturbation theory models with an Antisymmetric Product of 1-reference orbital Geminal (AP1roG) reference function that allows us to describe both static/nondynamic and dynamic electron correlation effects.

On the multi-reference nature of plutonium oxides: PuO, PuO, PuO and PuO(OH).

Actinide-containing complexes present formidable challenges for electronic structure methods due to the large number of degenerate or quasi-degenerate electronic states arising from partially occupied 5f and 6d shells. Conventional multi-reference methods can treat active spaces that are often at the upper limit of what is required for a proper treatment of species with complex electronic structures, leaving no room for verifying their suitability. In this work we address the issue of properly defining the active spaces in such calculations, and introduce a protocol to determine optimal active spaces based on the use of the Density Matrix Renormalization Group algorithm and concepts of quantum information theory.

Dissecting the cation-cation interaction between two uranyl units.

We present a state-of-the-art computational study of the uranyl(vi) and uranyl(v) cation-cation interactions (dications) in aqueous solution. Reliable electronic structures of two interacting uranyl(vi) and uranyl(v) subunits as well as those of the uranyl(vi) and uranyl(v) clusters are presented for the first time. Our theoretical study elucidates the impact of cation-cation interactions on changes in the molecular structure as well as changes in vibrational and UV-Vis spectra of the bare uranyl(vi) and uranyl(v) moieties for different total spin-states and total charges of the dications.

Nonvariational Orbital Optimization Techniques for the AP1roG Wave Function.

We introduce new nonvariational orbital optimization schemes for the antisymmetric product of one-reference orbital geminal (AP1roG) wave function (also known as pair-coupled cluster doubles) that are extensions to our recently proposed projected seniority-two (PS2-AP1roG) orbital optimization method [ J. Chem. Phys.

Singlet ground state actinide chemistry with geminals.

We present the first application of the variationally orbital optimized antisymmetric product of 1-reference orbital geminals (vOO-AP1roG) method to singlet-state actinide chemistry. We assess the accuracy and reliability of the AP1roG ansatz in modelling the ground-state electronic structure of small actinide compounds by comparing it to standard quantum chemistry approaches. Our study of the ground state spectroscopic constants (bond lengths and vibrational frequencies) and potential energy curves of actinide oxides (UO2(2+) and ThO2) as well as the energetic stability of ThC2 isomers reveals that vOO-AP1roG describes the electronic structure of heavy-element compounds accurately, at mean-field computational cost.

Communication: Relativistic Fock-space coupled cluster study of small building blocks of larger uranium complexes.

We present a study of the electronic structure of the [UO2](+), [UO2](2 +), [UO2](3 +), NUO, [NUO](+), [NUO](2 +), [NUN](-), NUN, and [NUN](+) molecules with the intermediate Hamiltonian Fock-space coupled cluster method. The accuracy of mean-field approaches based on the eXact-2-Component Hamiltonian to incorporate spin-orbit coupling and Gaunt interactions are compared to results obtained with the Dirac-Coulomb Hamiltonian. Furthermore, we assess the reliability of calculations employing approximate density functionals in describing electronic spectra and quantities useful in rationalizing Uranium (VI) species reactivity (hardness, electronegativity, and electrophilicity).

Projected seniority-two orbital optimization of the antisymmetric product of one-reference orbital geminal.

We present a new, non-variational orbital-optimization scheme for the antisymmetric product of one-reference orbital geminal wave function. Our approach is motivated by the observation that an orbital-optimized seniority-zero configuration interaction (CI) expansion yields similar results to an orbital-optimized seniority-zero-plus-two CI expansion [L. Bytautas, T.