Capturing Correlation Effects in Positron Binding to Atoms and Molecules.

A major challenge in contemporary electronic structure theory involves the development of methods to describe in a balanced manner the contribution of correlation effects to energy differences. This challenge can be even greater for multicomponent systems containing more than one type of quantum particle. In the present work, we describe a flexible code for carrying out self-consistent field and configuration interaction (CI) calculations on multicomponent systems and use it to generate trial wave functions for use in diffusion Monte Carlo (DMC) calculations of the positron affinity of Be, Be, Be, Mg, CS, and benzene.

Effect of N Atom Substitution on Electronic Resonances: A 2D Photoelectron Spectroscopic and Computational Study of Anthracene, Acridine, and Phenazine Anions.

The accommodation of an excess electron by polycyclic aromatic hydrocarbons (PAHs) has important chemical and technological implications ranging from molecular electronics to charge balance in interstellar molecular clouds. Here, we use two-dimensional photoelectron spectroscopy and equation-of-motion coupled-cluster calculations of the radical anions of acridine (CHN) and phenazine (CHN) and compare our results for these species to those for the anthracene anion (CH). The calculations predict the observed resonances and additionally find low-energy two-particle-one-hole states, which are not immediately apparent in the spectra, and offer a slightly revised interpretation of the resonances in anthracene.

Demonstrating the Connection between the Nonvalence Correlation-Bound Anions of Polyaromatic Hydrocarbons and the Image Potential States of Graphene Using a One-Electron Model Hamiltonian.

The ground and excited state nonvalence correlation-bound (NVCB) anion states of the hexagonal polycylic aromatic hydrocarbons and of hexagonal graphene nanoflakes are characterized using a one-electron model Hamiltonian which incorporates atomic electrostatic moments up to the quadrupole, coupled inducible charges and dipoles, and atom-centered Gaussians to describe the short-range repulsive interactions. Extrapolation of the calculated electron binding energies of the lowest energy symmetric and antisymmetric (with respect to the molecular plane) NVCB anions of both the polycylic aromatic hydrocarbons and the carbon nanoflakes to the → ∞ limit yields binding energies that are in good agreement with those of the most stable symmetric and antisymmetric image potential states of freestanding graphene as determined from two-photon photoemission spectroscopy (2PPE) experiments.

Contemporary Prophylactic Antibiotic Practices and Adjunct Therapies in Autologous Fat Grafting Procedures: A Survey of The Aesthetic Society Members.

Background: Autologous fat grafting (AFG) is a widely used surgical technique that involves extracting a patient's own adipose tissue and transferring it to different areas of the body. This practice is still evolving. Guidelines for antibiotic prophylaxis and use of adjuncts in plastic surgery are currently limited, with a notable absence of standardized guidelines for AFG.

Use of bound state methods to calculate partial and total widths of shape resonances.

In this work we study the Π resonances of a two-site model system designed to mimic a smooth transition from the Π temporary anion of N to the Π temporary anion of CO. The model system possesses the advantage that scattering and bound state () methods can be directly compared without obfuscating electron-correlation effects. Specifically, we compare resonance parameters obtained with the complex Kohn variational (CKV) method with those from stabilization, complex absorbing potential, and regularized analytical continuation calculations.

Application of a Fluctuating Charge Polarization Model to Large Polyaromatic Hydrocarbons and Graphene Nanoflakes.

We present a polarization model incorporating coupled fluctuating charges and point inducible dipoles that is able to accurately describe the dipole polarizabilities of small hydrocarbons and, for sufficiently large graphene nanoflakes, reproduce the classical image potential of an infinite conducting sheet. When our fluctuating charge model is applied to the hexagonal carbon nanoflake C we attain excellent agreement with the image potential and induced charge distribution of a conducting sheet. With the inclusion of inducible dipole terms, the model predicts an image plane of = 1.

Isotope Effects in the Zundel-Eigen Isomerization of H(HO).

The isomerization pathway between the energetically low-lying Zundel and Eigen isomers of the protonated water hexamer was investigated using high-level calculations including a treatment of zero-point corrections. On the basis of these calculations, the Zundel-Eigen isomerization was found to proceed through a stable intermediate isomer, which consists of a four-membered ring with two single acceptor water molecules. The inclusion of vibrational zero-point energy is shown to be important for accurately establishing the relative energies of the three relevant isomers involved in the Zundel-Eigen isomerization.

Crystallographic Mapping and Tuning of Water Adsorption in Metal-Organic Frameworks Featuring Distinct Open Metal Sites.

Crucial steps toward designing water sorption materials and fine-tuning their properties for specific applications include precise identification of adsorption sites and establishment of rigorous molecular-level insight into the water adsorption process. We report stepwise crystallographic mapping and density functional theory computations of adsorbed water molecules in ALP-MOF-1, a metal-organic framework decorated with distinct open metal sites and carbonyl functional groups that serve as water anchoring sites for seeding the nucleation of a complex water network. Identification of an unusual water adsorption step in ALP-MOF-1 motivated the tuning of metal ion composition to adjust water uptake.

Non-Valence Anions of Pyridine and the Diazines.

The dipole-bound anions of pyridine, pyridazine, and pyrimidine are characterized using equation of motion coupled cluster singles and doubles calculations. These calculations predict that the anions of pyridine, pyrimidine and pyridazine are bound in the Born-Oppenheimer approximation by 0.05, 0.

The binding of atomic hydrogen on graphene from density functional theory and diffusion Monte Carlo calculations.

In this work, density functional theory (DFT) and diffusion Monte Carlo (DMC) methods are used to calculate the binding energy of a H atom chemisorbed on the graphene surface. The DMC value of the binding energy is about 16% smaller in magnitude than the Perdew-Burke-Ernzerhof (PBE) result. The inclusion of exact exchange through the use of the Heyd-Scuseria-Ernzerhof functional brings the DFT value of the binding energy closer in line with the DMC result.

Water Network Shape-Dependence of Local Interactions with the Microhydrated -NO and -CO Anionic Head Groups by Cold Ion Vibrational Spectroscopy.

We report the structural evolutions of water networks and solvatochromic response of the CHNO radical anion in the OH and CH stretching regions by analysis of the vibrational spectra displayed by cryogenically cooled CHNO·(HO) clusters. The OH stretching bands evolve with a surprisingly large discontinuity at = 6, which features the emergence of an intense, strongly red-shifted band along with a weaker feature that appears in the region assigned to a free OH fundamental. Very similar behavior is displayed by the perdeuterated carboxylate clusters, RCO·(HO) (R = CDCD), indicating that this behavior is a general feature in the microhydration of the triatomic anionic domain and not associated with CH oscillators.

Vibrational Signatures of HNO Acidity When Complexed with Microhydrated Alkali Metal Ions, M·(HNO)(HO) (M = Li, K, Na, Rb, Cs), at 20 K.

The speciation of strong acids like HNO under conditions of restricted hydration is an important factor in the rates of chemical reactions at the air-water interface. Here, we explore the trade-offs at play when HNO is attached to alkali ions (Li-Cs) with four water molecules in their primary hydration shells. This is achieved by analyzing the vibrational spectra of the M·(HNO)(HO) clusters cooled to about 20 K in a cryogenic photofragmentation mass spectrometer.

Real-Time Modulation of Hydrogen Evolution Activity of Graphene Electrodes Using Mechanical Strain.

This paper reports the effect of mechanically applied elastic strain on the hydrogen evolution reaction (HER) activity of graphene under acidic conditions. An applied tensile strain of 0.2% on a graphene electrode is shown to lead to a 1-3% increase in the HER current.

Temporary Anion Resonances of Pyrene: A 2D Photoelectron Imaging and Computational Study.

The low-energy electron-scattering resonances of pyrene were characterized using experimental and computational methods. Experimentally, a two-dimensional photoelectron imaging of the pyrene anion was used to probe the dynamics of resonances over the first 4 eV of the continuum. Computationally, the energies and character of the anion states were determined using equation-of-motion coupled cluster calculations, while taking specific care to avoid the collapse onto discretized continuum levels, and an application of the pairing theorem.

Frontiers of stochastic electronic structure calculations.

In recent years there has been a rapid growth in the development and application of new stochastic methods in electronic structure. These methods are quite diverse, from many-body wave function techniques in real space or determinant space to being used to sum perturbative expansions. This growth has been spurred by the more favorable scaling with the number of electrons and often better parallelization over large numbers of central processing unit (CPU) cores or graphical processing units (GPUs) than for high-end non-stochastic wave function based methods.

Two-Dimensional Adiabatic Model for Calculating Progressions Resulting from Stretch-Rock Coupling in Vibrational Spectra of Anion-Water Complexes.

Several anion-water dimers feature a distinct progression in the OH stretch region of their vibrational spectra. This progression arises from strong anharmonic couplings between the OH stretch and low-frequency intermolecular modes. In this work, we introduce a two-dimensional adiabatic model accounting explicitly for the water and anion rock degrees of freedom and use it to calculate the vibrational spectra of HCO·(HO) and NO·(HO).

Role of Overlap between the Discrete State and Pseudocontinuum States in Stabilization Calculations of Metastable States.

In a diabatic picture metastable states subject to decay by electron detachment can be viewed as arising from the coupling between a discrete state and a continuum. In treating such states with bound-state quantum chemical methods, the continuum is discretized. In this study, we elucidate the role of overlap in this interaction in the application of the stabilization method to temporary anion states.

A Fresh Look at the Role of the Coupling of a Discrete State with a Pseudocontinuum State in the Stabilization Method for Characterizing Metastable States.

The stabilization method is widely used to theoretically characterize temporary anions and other systems displaying resonances. In this approach, information about a metastable state is encoded in the interaction of a diabatic discrete state and discretized continuum solutions, the energy of which are varied by scaling the extent of the basis set. In this work, we identify the aspects of the coupling between the discrete state and the discretized continuum states that encode information about the existence of complex stationary points and, hence, complex resonance energies in stabilization graphs.

The role of high-order electron correlation effects in a model system for non-valence correlation-bound anions.

The diffusion Monte Carlo (DMC), auxiliary field quantum Monte Carlo (AFQMC), and equation-of-motion coupled cluster (EOM-CC) methods are used to calculate the electron binding energy (EBE) of the non-valence anion state of a model (HO) cluster. Two geometries are considered, one at which the anion is unbound and the other at which it is bound in the Hartree-Fock (HF) approximation. It is demonstrated that DMC calculations can recover from the use of a HF trial wave function that has collapsed onto a discretized continuum solution, although larger EBEs are obtained when using a trial wave function for the anion that provides a more realistic description of the charge distribution and, hence, of the nodal surface.

Toward a systematic improvement of the fixed-node approximation in diffusion Monte Carlo for solids-A case study in diamond.

While Diffusion Monte Carlo (DMC) is in principle an exact stochastic method for ab initio electronic structure calculations, in practice, the fermionic sign problem necessitates the use of the fixed-node approximation and trial wavefunctions with approximate nodes (or zeros). This approximation introduces a variational error in the energy that potentially can be tested and systematically improved. Here, we present a computational method that produces trial wavefunctions with systematically improvable nodes for DMC calculations of periodic solids.

Mapping the temperature-dependent and network site-specific onset of spectral diffusion at the surface of a water cluster cage.

We explore the kinetic processes that sustain equilibrium in a microscopic, finite system. This is accomplished by monitoring the spontaneous, time-dependent frequency evolution (the frequency autocorrelation) of a single OH oscillator, embedded in a water cluster held in a temperature-controlled ion trap. The measurements are carried out by applying two-color, infrared-infrared photodissociation mass spectrometry to the DO·(HDO)(DO) isotopologue of the "magic number" protonated water cluster, H·(HO) The OH group can occupy any one of the five spectroscopically distinct sites in the distorted pentagonal dodecahedron cage structure.

QMCPACK: Advances in the development, efficiency, and application of auxiliary field and real-space variational and diffusion quantum Monte Carlo.

We review recent advances in the capabilities of the open source ab initio Quantum Monte Carlo (QMC) package QMCPACK and the workflow tool Nexus used for greater efficiency and reproducibility. The auxiliary field QMC (AFQMC) implementation has been greatly expanded to include k-point symmetries, tensor-hypercontraction, and accelerated graphical processing unit (GPU) support. These scaling and memory reductions greatly increase the number of orbitals that can practically be included in AFQMC calculations, increasing the accuracy.

Analysis of the Contributions to the Kinetic and Potential Energies of an H Atom in the Presence of a Point Charge: The Molecular Virial Theorem Revisited.

The molecular virial theorem states that for a diatomic molecule or for an atom in the presence of a point charge, the changes in the average kinetic energy and average potential energy are equal to [Formula: see text] and [Formula: see text], respectively, where is the interaction energy and is the internuclear separation or the atom-point charge separation. In this paper we directly evaluate the ⟨⟩ and ⟨⟩ expectation values of an H atom in the presence of a distant point charge, obtaining exact analytical expressions by use of Dalgarno-Lewis perturbation theory.