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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. The tensile strain increases HER activity, whereas compressive strain decreases it. Density functional theory (DFT) calculations using a periodic graphene slab model predict an increase in the adsorption energy of the H atom with growing tensile strain, consistent with an enhancement of the current density in HER, similar to that observed experimentally.

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. These studies provide direct evidence that the identity of the open metal sites in MOFs can dramatically affect water adsorption behavior between 0 and ∼20% RH and that multiple proximal water anchoring sites along the MOF skeleton facilitate water uptake which could be potentially useful for applications requiring rapid and energetically facile water sorption.

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. Our results are in good agreement with the predictions of electron-scattering calculations that include an offset and with the pyrene anion absorption spectrum in a glass matrix. Taken together, we offer an assignment of the first five electronic resonances of pyrene. Some of the population in the lowest-energy 2B1u resonance was observed to decay to the ground electronic state of the anion, while all other resonances decay by a direct autodetachment. The astronomical relevance of a ground-state electron capture proceeding via a low-energy resonance in pyrene is discussed.

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 HCO2-·(H2O) and NO3-·(H2O). The spectra calculated by using this model are in excellent agreement with experiment, in terms of both peak spacings and lengths of the progressions, and represent a substantial improvement over earlier models.

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, 0.8, and 19.0 meV, respectively. The binding energies of pyrimidine and pyridazine are large enough that the anions will remain bound even when allowing for corrections to the Born-Oppenheimer approximation, while that of pyridine is a borderline case. We were unable to find a stable non-valence correlation-bound anion for pyrazine, which has a zero dipole moment.

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. This is accomplished by use of a minimalist stabilization calculation on the lowest energy l=2 (D) resonance of the finite spherical well potential using two basis functions, one describing the diabatic discrete state and the other a diabatic discretized continuum state. We show that even such a simple treatment predicts a complex resonance energy in good agreement with the exact result. If the energy of the discrete state is assumed to be constant, which is tantamount to orthogonalizing the discretized continuum state to the discrete state, it is demonstrated that the square of the off-diagonal coupling has a maximum close to the crossing point of the orthogonalized diabatic curves and that the curvature in the coupling is responsible for the complex stationary point associated with the resonance. Moreover, this curvature is a consequence of the overlap between the two diabatic states.

We report the structural evolutions of water networks and solvatochromic response of the CH

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

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 Perdew-Burke-Ernzerhof (PBE) value of the binding energy is about 20% larger in magnitude than the diffusion Monte Carlo result. The inclusion of exact exchange through the use of the Heyd-Scuseria-Ernzerhof (HSE) functional brings the DFT value of the binding energy closer in line with the DMC result. It is also found that there are significant differences in the charge distributions determined using PBE and DMC approaches., Comment: The following article has been submitted to The Journal of Chemical Physics

Ab initio calculations are used to demonstrate the existence of a nonvalence temporary anion shape resonance for a model (H2O)4 cluster system with no net dipole moment. The cluster is composed of ...

A polarization model which accounts for electric field-induced charge transfer between fullerene molecules is introduced. Application of this model to the C60 dimer and trimer shows that intermolecular charge transfer makes a significant contribution to the polarizabilities of these clusters. This polarization model is incorporated into a one-electron Hamiltonian for describing non-valence correlation-bound anions, allowing us to further demonstrate that intermolecular charge transfer also results in increased stability of these anion states.

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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. This allows us to design a simple two-level model for extracting complex resonance energies from stabilization graphs. The resulting model is applied to the

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 (H$_2$O)$_4$ 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 electron binding energies 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. For the geometry at which the cluster has a non-valence correlation-bound anion, both the inclusion of triples in the EOM-CC method and the inclusion of supplemental diffuse d functions in the basis set are important. DMC calculations with suitable trial wave functions give EBE values in good agreement with our best estimate EOM-CC result. AFQMC using a trial wave function for the anion with a realistic electron density gives a value of the EBE nearly identical to the EOM-CC result when using the same basis set. For the geometry at which the anion is bound in the HF approximation, the inclusion of triple excitations in the EOM-CC calculations is much less important. The best estimate EOM-CC EBE value is in good agreement with the results of DMC calculations with appropriate trial wave functions., Comment: The following article has been submitted to The Journal of Chemical Physics

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 D(3)O(+)·(HDO)(D(2)O)(19) isotopologue of the “magic number” protonated water cluster, H(+)·(H(2)O)(21). The OH group can occupy any one of the five spectroscopically distinct sites in the distorted pentagonal dodecahedron cage structure. The OH frequency is observed to evolve over tens of milliseconds in the temperature range (90 to 120 K). Starting at 100 K, large “jumps” are observed between two OH frequencies separated by ∼300 cm(−1), indicating migration of the OH group from the bound OH site at 3,350 cm(−1) to the free position at 3,686 cm(−1). Increasing the temperature to 110 K leads to partial interconversion among many sites. All sites are observed to interconvert at 120 K such that the distribution of the unique OH group among them adopts the form one would expect for a canonical ensemble. The spectral dynamics displayed by the clusters thus offer an unprecedented view into the molecular-level processes that drive spectral diffusion in an extended network of water molecules.

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. Advances in real space methods include techniques for accurate computation of bandgaps and for systematically improving the nodal surface of ground state wavefunctions. Results of these calculations can be used to validate application of more approximate electronic structure methods, including GW and density functional based techniques. To provide an improved foundation for these calculations, we utilize a new set of correlation-consistent effective core potentials (pseudopotentials) that are more accurate than previous sets; these can also be applied in quantum-chemical and other many-body applications, not only QMC. These advances increase the efficiency, accuracy, and range of properties that can be studied in both molecules and materials with QMC and QMCPACK. publishedVersion

An exactly soluble model of two diabatic electronic states interacting through a coupling of strength V is used to generate data for testing the rational fraction analytic continuation technique for determining the energies and widths of metastable states of anions. By making analytical connections between the coefficients in the rational fraction and the parameters of the model, we are able to suggest how to choose the orders of the polynomials and the range of the scaling parameter, Z, within which to compute the energies for a given precision. This analysis also allows us to specify the range of Z-values to use in constructing the rational fraction in a manner that allows determination of all parameters of the model for a given precision. The constraint on the Z-value ranges can be used as a guide for constructing rational fractions of data obtained in electronic structure calculations on actual resonance states.

We report vibrational spectra of the cryogenically cooled H9O4+ cation along with those of the D2 tagged HD8O4+ isotopomers using two variations on a two-color, IR–IR double-resonance photoexcitation scheme. The spectrum of the isolated H9O4+ ion consists of two sharp features in the OH stretching region that indicate exclusive formation of the “Eigen” cation, the H3O+·(H2O)3 isomer that corresponds to the filled hydration shell around the hydronium ion. Consistent with this structural assignment, the spectrum of the HD8O4+ isotopologue is resolved into contributions from two isotopomers: one with the single OH group on one of the three solvent water molecules and another in which it resides on the hydronium core ion. The latter spectrum is dominated by a broad feature assigned to the isolated hydronium OH stretching fundamental with an envelope that is similar to that displayed by the H3O+·(H2O)3 isotopologue. The feature appears with a diffuse band ∼380 cm–1 above it, which is assigned to a combination ...

The stabilization method is used in conjunction with the equation-of-motion electron-attachment coupled-cluster method to calculate the complex energies of the 2 P temporary anion states of Be and Mg as well as of the 2 D temporary anions states of Mg and Ca. The calculated resonance parameters for the 2 P state of Mg − and 2 D state of Ca − agree well with experiment. Experimental results are not available for the 2 P anion of Be, but we note that our calculated resonance parameters of 2 P Be − , while in good agreement with the CI results of McNutt and McCurdy (1983), differ significantly from the results of two other recent theoretical studies.

The article by Moberg et al. (1) addresses the fundamental question of the number of molecules a water cluster must contain in order to find a significant fraction of the molecules with a hydrogen-bonding arrangement characteristic of ice I, which is a mixture of the structurally similar hexagonal ice Ih and cubic Ic species (2). An early experimental study addressing this question placed the critical cluster size for displaying ice-like behavior between 200 and 1,000 molecules (3). More recent estimates of the smallest water cluster with ice I character range from about 90 to 300 molecules (2, 4⇓⇓–7). The study of Moberg et al. brings to bear experiments and computer simulations to obtain a more precise estimate of the cluster size at which ice I character is manifested and to elucidate the factors responsible for the wide spread in earlier estimates of the critical cluster size. The preparation of isolated water clusters in the gas phase is generally accomplished by using seeded supersonic jet expansion of a mixture of water and a buffer gas. Such expansions start at relatively high temperatures and pressures, with both temperature and pressure decreasing as the expansion proceeds. Depending on the conditions employed in the expansion, the cooling rate can vary appreciably, which can impact the resulting structures. If the cooling rate is too fast, the resulting clusters can be trapped in local energy minima and, thus, not sample thermodynamically more stable structures. In other words, rapid cooling could cause the experiments to sample clusters that are amorphous in nature while, in fact, the crystalline form could be more stable at the final temperature achieved. In the experiments of Moberg et al. (1), the expansion conditions are varied to achieve slow as well as fast cooling (with low stagnation pressure … [↵][1]1Email: jordan{at}pitt.edu. [1]: #xref-corresp-1-1

The equation-of-motion coupled cluster method is used to characterize the low-lying anion states of (NaCl)

Many water-anion complexes of the form X-·(H2O), where X- is a polyatomic anion, display a peak progression in the OH stretch region of the vibrational spectra with spacings of 65-85 cm-1. These progressions result from strong anharmonic coupling between the OH stretch and a low-frequency intermolecular rock vibration. In this study, we calculate these progressions in HCO2-·(H2O), NO3-·(H2O), and CS2-·(H2O) by use of a one-dimensional adiabatic model with rock potentials generated from ab initio energies and frequencies. The importance of using a geometry-dependent reduced mass in calculating the peak spacings is demonstrated. We find that the one-dimensional adiabatic model is more successful in predicting peak spacings in the spectrum of HCO2-·(H2O) than for NO3-·(H2O), for which the rock vibration is highly anharmonic.

We exploit gas-phase cluster ion techniques to provide insight into the local interactions underlying divalent metal ion-driven changes in the spectra of carboxylic acids at the air–water interface. This information clarifies the experimental findings that the CO stretching bands of long-chain acids appear at very similar energies when the head group is deprotonated by high subphase pH or exposed to relatively high concentrations of Ca 2+ metal ions. To this end, we report the evolution of the vibrational spectra of size-selected [Ca 2+ ·RCO 2 − ] + ·(H 2 O) n =0 to 12 and RCO 2 − ·(H 2 O) n =0 to 14 cluster ions toward the features observed at the air–water interface. Surprisingly, not only does stepwise hydration of the RCO 2 − anion and the [Ca 2+ ·RCO 2 − ] + contact ion pair yield solvatochromic responses in opposite directions, but in both cases, the responses of the 2 (symmetric and asymmetric stretching) CO bands to hydration are opposite to each other. The result is that both CO bands evolve toward their interfacial asymptotes from opposite directions. Simulations of the [Ca 2+ ·RCO 2 − ] + ·(H 2 O) n clusters indicate that the metal ion remains directly bound to the head group in a contact ion pair motif as the asymmetric CO stretch converges at the interfacial value by n = 12. This establishes that direct metal complexation or deprotonation can account for the interfacial behavior. We discuss these effects in the context of a model that invokes the water network-dependent local electric field along the C–C bond that connects the head group to the hydrocarbon tail as the key microscopic parameter that is correlated with the observed trends.

A polarization model which accounts for electric field-induced charge transfer between fullerene molecules is introduced. Application of this model to the C60 dimer and trimer shows that intermolecular charge transfer makes a significant contribution to the polarizabilities of these clusters. This polarization model is incorporated into a one-electron Hamiltonian for describing non-valence correlation-bound anions, allowing us to further demonstrate that intermolecular charge transfer also results in increased stability of these anion states.

Ab initio calculations are used to demonstrate the existence of a nonvalence temporary anion shape resonance for a model (H

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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. This special issue of the Journal of Chemical Physics includes 33 papers that describe recent developments and applications in this area. As seen from the articles in the issue, stochastic electronic structure methods are applicable to both molecules and solids and can accurately describe systems with strong electron correlation. This issue was motivated, in part, by the 2019 Telluride Science Research Center workshop on Stochastic Electronic Structure Methods that we organized. Below we briefly describe each of the papers in the special issue, dividing the papers into six subtopics.

The Grotthuss mechanism explains the anomalously high proton mobility in water as a sequence of proton transfers along a hydrogen-bonded (H-bonded) network. However, the vibrational spectroscopic signatures of this process are masked by the diffuse nature of the key bands in bulk water. Here we report how the much simpler vibrational spectra of cold, composition-selected heavy water clusters, D+(D2O)n, can be exploited to capture clear markers that encode the collective reaction coordinate along the proton-transfer event. By complexing the solvated hydronium “Eigen” cluster [D3O+(D2O)3] with increasingly strong H-bond acceptor molecules (D2, N2, CO, and D2O), we are able to track the frequency of every O-D stretch vibration in the complex as the transferring hydron is incrementally pulled from the central hydronium to a neighboring water molecule.

Using scanning tunneling microscopy and electronic structure calculations, we investigated the assembly and electronic properties of picene molecules on the Ag(100), Ag(111), and Cu(111) surfaces, with particular emphasis on Ag(100). In each case, picene molecules are found to lie parallel to the surface at coverages up to half a monolayer and to adopt alternating parallel and tilted orientations at full monolayer coverage. In the latter case, the arrangement of the molecules is roughly similar to that in the bulk crystal. On the metal surfaces considered, the growth mode of picene is quite different from that of its structural isomer pentacene, which forms a bilayer overlayer on top of a dense monolayer of flat-lying molecules on metal surfaces. Tunneling spectroscopy measurements provide estimates of the energies of several low-lying unfilled molecular orbitals as well as of the highest occupied molecular orbital of the absorbed picene molecules. From analysis of these results, we establish that the on-site Coulomb repulsion for picene decreases by ~ 2 eV in going from the gas phase to the full monolayer on Ag(100), bringing it close to that of the undoped bulk crystal.

The equation-of-motion method is used to map out one-dimensional potentials for proton-coupled electron transfer in the [pyridine·(H 2 O) n ] − , n = 3, 4, clusters. In both cases there is an avoided crossing between valence-type pyridinyl·(OH − ) (H 2 O) n −1 and dipole–bound pyridine·(H 2 O) n − anion species. For the n = 3 cluster, the mixing is sufficiently strong that both limiting structures are predicted to be sampled in the vibrational zero-point level.

This study explores the utility of Brueckner orbitals as trial wave functions for diffusion Monte Carlo (DMC) calculations. Comparison is made with Hartree⿿Fock (HF) and density functional theory (DFT) orbitals allowing for an assessment of how well the three sets of orbitals describe the nodal surfaces. For the neutral test systems, PBE0 orbitals or Brueckner orbitals give DMC energies that are appreciably lower than those obtained using Hartree⿿Fock orbitals. For a CO 2 ⿿ anion test case, a significantly lower DMC energy is obtained when using Brueckner orbitals rather than DFT orbitals as the trial function.

MP2 calculations with the full aug-cc-pVTZ basis set give a non-planar structure for benzene. Although this non-physical result can be avoided by using the smaller aug-cc-pVDZ basis set or by scaling or deleting selected functions from the aug-cc-pVTZ basis set, such changes to the basis set can result in calculated values of the frequencies of the b2g out-of-plane vibrations that are considerably underestimated. The origin of this behavior is traced to linear dependency problems with the aug-cc-pVDZ and aug-cc-pVTZ basis sets when used for benzene.

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. These trial wavefunctions are efficiently generated with the configuration interaction using a perturbative selection made iteratively (CIPSI) method. A simple protocol in which both exact and approximate results for finite supercells are used to extrapolate to the thermodynamic limit is introduced. This approach is illustrated in the case of the carbon diamond using Slater-Jastrow trial wavefunctions including up to one million Slater determinants. Fixed-node DMC energies obtained with such large expansions are much improved, and the fixed-node error is found to decrease monotonically and smoothly as a function of the number of determinants in the trial wavefunction, a property opening the way to a better control of this error. The cohesive energy extrapolated to the thermodynamic limit is in close agreement with the estimated experimental value. Interestingly, this is also the case at the single-determinant level, thus, indicating a very good error cancellation in carbon diamond between the bulk and atomic total fixed-node energies when using single-determinant nodes.

We report vibrational spectra of the cryogenically cooled H

Molecular dynamics simulations using classical force fields were carried out to study energetic and structural properties of rotationally disordered clay mineral-water-CO2 systems at pressure and temperature relevant to geological carbon storage. The simulations show that turbostratic stacking of hydrated Na- and Ca-montmorillonite and hydrated montmorillonite with intercalated carbon dioxide is an energetically demanding process accompanied by an increase in the interlayer spacing. On the other hand, rotational disordering of dry or nearly dry smectite systems can be energetically favorable. The distributions of interlayer species are calculated as a function of the rotational angle between adjacent clay layers.

Neutral molecules with sufficiently large dipole moments can bind electrons in diffuse nonvalence orbitals with most of their charge density far from the nuclei, forming so-called dipole-bound anions. Because long-range correlation effects play an important role in the binding of an excess electron and overall binding energies are often only on the order of 10s–100s of wave numbers, predictively modeling dipole-bound anions remains a challenge. Here, we demonstrate that quantum Monte Carlo methods can accurately characterize molecular dipole-bound anions with near-threshold dipole moments. We also show that correlated sampling Auxiliary Field Quantum Monte Carlo is particularly well-suited for resolving the fine energy differences between the neutral and anionic species. These results shed light on the fundamental limitations of quantum Monte Carlo methods and pave the way toward using them for the study of weakly bound species that are too large to model using traditional electron structure methods.

Multiphase Gibbs ensemble Monte Carlo simulations were carried out to compute the free energy of swelling for Na-montmorillonite and Na-beidellite interacting with CO2 and H2O at pressure and temperature conditions relevant for geological storage aquifers. The calculated swelling free energy curves show stable monolayer and bilayer configurations of the interlayer species for Na-montmorillonite, while only the monolayer structure is found to be stable for Na-beidellite. The calculations show that CO2 is intercalated into hydrated clay phases at concentrations greatly exceeding its solubility in bulk water. This suggests that expandable clay minerals are good candidates for storing carbon dioxide in interlayer regions. For Na-beidellite the CO2 molecule distribution is mainly controlled by the position of the isomorphic substitutions, while for Na-montmorillonite the hydrated sodium ions play an important role in establishing the CO2 distribution.

The configuration interaction, perturbation theory, coupled cluster doubles, and random phase approximation methods are applied to a system of two quantum Drude oscillators interacting through dipole–dipole coupling. It is found that the random phase approximation gives the exact excitation energies and exact interaction energy of this system even when allowing only excitations into the first excited levels of the oscillators. In contrast, to obtain the exact results from configuration interaction or coupled cluster treatments of the model requires inclusion of excitations into all accessible excited configurations.

Isotopic exchange of oxygen atoms between gaseous C18O2 and Ti16O2 nanoparticles has been studied using high-resolution Fourier transform infrared absorption and first-principles density functional theory calculations. The rate of formation of gaseous C16O2 is found to be highly dependent on the nature of the titania sample, growing with increasing calcination temperature (i.e., with decreasing surface area) for both quasi-amorphous and crystalline samples. The unprecedented faster kinetics on high-surface-area titania made from Ti(IV) isopropoxide points at fundamental differences between this material and the usual nanocrystalline TiO2 (anatase). This is attributed to unique cluster-like structure of the noncalcined ex-isopropoxide titania. The experimental observations are rationalized by calculations of the activation barriers for oxygen exchange on a (TiO2)10 cluster. The calculations predict that titanium nanoclusters with 4-fold coordinated titanium atoms have much lower barriers for O atom exchang...

This Special Topic issue entitled "From Quantum Mechanics to Force Fields" is dedicated to the ongoing efforts of the theoretical chemistry community to develop a new generation of accurate force fields based on data from high-level electronic structure calculations and to develop faster electronic structure methods for testing and designing force fields as well as for carrying out simulations. This issue includes a collection of 35 original research articles that illustrate recent theoretical advances in the field. It provides a timely snapshot of recent developments in the generation of approaches to enable more accurate molecular simulations of processes important in chemistry, physics, biophysics, and materials science.

A model H