Algorithmic differentiation and the calculation of forces by quantum Monte Carlo

We describe an efficient algorithm to compute forces in quantum Monte Carlo using adjoint algorithmic differentiation. This allows us to apply the space warp coordinate transformation in differential form, and compute all the 3M force components of a system with M atoms with a computational effort comparable with the one to obtain the total energy. Few examples illustrating the method for an electronic system containing several water molecules are presented. With the present technique, the calculation of finite-temperature thermodynamic properties of materials with quantum Monte Carlo will be feasible in the near future.     

Density functional theory and pseudopotentials: A panacea for calculating properties of materials

Although our microscopic view of solids is still evolving, for a large class of materials one can construct a useful first principles or “standard model” of solids which is sufficiently robust to explain and predict many physical properties. Both electronic and structural properties can be studied, and the results of the first-principles calculations can be used to predict new materials, formulate empirical theories and simple formulas to compute material parameters, and explain trends. A discussion of the microscopic approach, applications, and empirical theories is given here, and some recent results on nanotubes, hard materials, and fullerenes are presented. © 1997 John Wiley & Sons, Inc.     

Probing gas adsorption in MOFs using an efficient ab initio widom insertion Monte Carlo method

We propose a novel biased Widom insertion method that can efficiently compute the Henry coefficient, K_H, of gas molecules inside porous materials exhibiting strong adsorption sites by employing purely DFT calculations. This is achieved by partitioning the simulation volume into strongly and weakly adsorbing regions and selectively biasing the Widom insertion moves into the former region. We show that only few thousands of single point energy calculations are necessary to achieve accurate statistics compared to many hundreds of thousands or millions of such calculations in conventional random insertions. The methodology is used to compute the Henry coefficient for CO₂, N₂, CH₄, and C₂H₂ in M‐MOF‐74(M = Zn and Mg), yielding good agreement with published experimental data. Our results demonstrate that the DFT binding energy and the heat of adsorption are not accurate enough indicators to rank the guest adsorption properties at the Henry regime. © 2016 Wiley Periodicals, Inc.     

The third industrial fluid properties simulation challenge

The third industrial fluid properties simulation challenge was held from March to September 2006. As in the previous two events contestants were challenged to predict specific, industrially relevant, properties of fluid systems. Their efforts were judged based on the agreement of the predicted values with previously unpublished experimental data (provided by researchers at ExxonMobil and DuPont). The focus of this contest was on the transferability of modeling methods—the ability to predict properties for materials that are chemically different, or at different state points, to those used in model parameterization and validation. Nine groups attempted to compute bubble point pressures for mixtures of 1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea) and ethanol at 343 K, given data for mixtures at 283 K, and given the pure component vapor pressures. They used a range of different techniques including statistical mechanical and molecular simulations-based approaches. Four of the groups were recognized for providing predictions that were significantly more accurate than would be obtained by extrapolation using the NRTL model (the standard engineering approach). Three groups undertook the more challenging “molecular transferability” problem, attempting to predict shear viscosities at two different state points for a range of diols and triols for which little experimental data was available.     

Chemical thermodynamics of polyaromatic compounds containing heteroatoms and five-membered rings

An analysis of chemical thermodynamic properties of gaseous polynuclear aromatic compounds containing N-, S-, and O-heteroatoms and five-membered rings is presented. Standard enthalpies of formation, entropies and heat capacities taken from the literature, or derived from appropriate data, serve as the data base for this analysis. Enthalpies of formation are used to compute empirical resonance energies for polynuclear heteroaromatic structures which, in turn, allow critical examination of this property. An additive method is described for estimating entropies and heat capacities. New or revised values for thermodynamic properties for a number of polyaromatic substances are presented and recommended values for some common polyaromatics are given.     

Science-Driven Atomistic Machine Learning

Machine learning (ML) algorithms are currently emerging as powerful tools in all areas of science. Conventionally, ML is understood as a fundamentally data-driven endeavour. Unfortunately, large well-curated databases are sparse in chemistry. In this contribution, I therefore review science-driven ML approaches which do not rely on “big data”, focusing on the atomistic modelling of materials and molecules. In this context, the term science-driven refers to approaches that begin with a scientific question and then ask what training data and model design choices are appropriate. As key features of science-driven ML, the automated and purpose-driven collection of data and the use of chemical and physical priors to achieve high data-efficiency are discussed. Furthermore, the importance of appropriate model evaluation and error estimation is emphasized.     

Molecular‐Weight Multimodality of Multiple Flory Distributions

It is well‐known that the final end‐use properties of polymer resins depend on the shape of the molecular‐weight distribution (MWD) very strongly. Particularly, polymer resins with bimodal MWDs are required for certain special applications, as they may simultaneously present enhanced mechanical and flow properties. A theoretical framework for the characterization of bimodality (or multimodality) of MWDs of polymers produced through linear polymerizations at steady‐state or quasi‐steady‐state conditions is developed and presented here. Conditions for the development of bimodality in generalized NS‐Schulz–Flory distributions are characterized for different forms of presentation of the MWDs. It is shown that the bimodal character of the MWD depends on the particular form used to represent it, which can then be used to generate an index of bimodality of the MWD. The theoretical results are finally used to compute the index of bimodality of actual polymer materials obtained at plant site.     

Materials design for perovskite SOFC cathodes

Abstract  

This article focuses on perovskite materials for application as cathode material in solid oxide fuel cells. In order to develop new promising materials it is helpful to classify already known perovskite materials according to their properties and to identify certain tendencies. Thereby, composition-dependent structural data and materials properties are considered. Structural data under consideration are the Goldschmidt tolerance factor, which describes the stability of perovskites with respect to other structures, and the critical radius and lattice free volume, which are used as geometrical measures of ionic conductivity. These calculations are based on the ionic radii of the constituent ions and their applicability is discussed. A potential map of perovskites as a tool to classify simple ABO₃ perovskite materials according to their electrical conduction behavior is critically reviewed as a structured approach to the search for new cathode materials based on more complex perovskites with A and/or B-site substitutions. This article also covers the approaches used to influence electronic and the ionic conductivity. The advantage of mixed ionic electronic conductors in terms of the oxygen exchange reaction is addressed and their important properties, namely the oxygen-exchange coefficient and the oxygen diffusion coefficient, and their effect on the oxygen reduction reaction are presented.     

Modeling flexible amphiphilic bilayers: a solvent-free off-lattice Monte Carlo study

We present a simple, implicit-solvent model for fluid bilayer membranes. The model was designed to reproduce the elastic properties of real bilayer membranes. For this model, we observed the solid-fluid transition and studied the in-plane diffusivity of the fluid phase. As a test, we compute the elastic-bending and area-compressing moduli of fluid bilayer membranes. We find that the computed elastic properties are consistent with the available experimental data.     

A second-order perturbation theory route to vibrational averages and transition properties of molecules: general formulation and application to infrared and vibrational circular dichroism spectroscopies

A general formulation to compute anharmonic vibrational averages and transition properties at the second-order of perturbation theory is derived from the Rayleigh-Schro?dinger development. This approach is intended to be applicable to any property expanded as a Taylor series up to the third order with respect to normal coordinates or their associated momenta. The equations are straightforward to implement and can be easily adapted to various properties, as illustrated for the case of electric and magnetic dipole moments. From those, infrared and vibrational circular dichroism spectra can be readily obtained. This fully automatic procedure has been applied to several chiral molecules of small-to-medium sizes and compared to the standard double harmonic approximation and to experimental data.     

Structure,epr parameters,and reactivity of organic free radicals from a density functional approach

Summary A recently introduced density functional incorporating gradient corrections and some Hartree-Fock exchange has been used to study the structures, properties, and reactivity of representative organic free radicals. A general theoretical model has been introduced, in which standardized grid, functional, and orbital basis set are used to compute geometrical parameters, vibrational frequencies, and one-electron properties. The results are compared with available experimental data from diatomic to polyatomic radicals. All the geometric and electronic parameters compare favourably with available experimental data and with the results of refined post Hartree-Fock computations. Also the thermodynamics and kinetics of a representative unimolecular reaction (isomerization of formaldehyde radical cation) are well reproduced. These findings together with the very favourable scaling of the computations with the number of electrons suggest that the density functional approach is a promising theoretical tool for the study of relationships between structure and properties of large free radicals.     

Materials design for perovskite SOFC cathodes

Abstract  This article focuses on perovskite materials for application as cathode material in solid oxide fuel cells. In order to develop new promising materials it is helpful to classify already known perovskite materials according to their properties and to identify certain tendencies. Thereby, composition-dependent structural data and materials properties are considered. Structural data under consideration are the Goldschmidt tolerance factor, which describes the stability of perovskites with respect to other structures, and the critical radius and lattice free volume, which are used as geometrical measures of ionic conductivity. These calculations are based on the ionic radii of the constituent ions and their applicability is discussed. A potential map of perovskites as a tool to classify simple ABO₃ perovskite materials according to their electrical conduction behavior is critically reviewed as a structured approach to the search for new cathode materials based on more complex perovskites with A and/or B-site substitutions. This article also covers the approaches used to influence electronic and the ionic conductivity. The advantage of mixed ionic electronic conductors in terms of the oxygen exchange reaction is addressed and their important properties, namely the oxygen-exchange coefficient and the oxygen diffusion coefficient, and their effect on the oxygen reduction reaction are presented. Graphical abstract        

MCSCF calculation of response properties of Argon

Summary Analytic equations of the multiconfigurational SCF (MCSCF) response theory are combined with the finite-field (FF) approach to compute static and frequency dependent electric and magnetic properties of the Argon atom. A complete active space (CAS SCF) function including the 3s, 3p, 3d, 4s and 4p orbitals in the active space and a large (17s 13p 7d 5f 3g) basis set are employed. This permits an accurate determination of various linear and non-linear response properties such as e.g. electric dipole polarisability and second hyperpolarisability, Verdet constant, magnetisability and second hyperpolarisability. The results, both for the static values and for the frequency dependence of these properties, compare well with other most recent experimental and theoretical data.Dedicated to Jan Linderberg on the occasion of his 60th birthday     

Influence of thermophysical properties on burning behavior of intumescent fire-retardant materials

Thermophysical properties of intumescent fire-retardant (IFR) materials are important parameters as input data in modeling the combustion process of IFR materials in a fire. In this paper, the influences of several thermophysical properties on burning behavior of IFR materials are simulated based on a combustion model of IFR materials. Thermophysical properties selected here are thermal conductivity of virgin material and char layer, specific heat capacity of virgin material, density of virgin material, surface emissivity of virgin material and char layer, heat of decomposition, heat of combustion, and intumescent temperature. Predicted heat release rates curves for the IFR material at an incident heat flux of 50 kW m^?2 are shown for the varied thermophysical parameters’ values. The results show that these varied parameter values can affect the burning behavior of materials remarkably. A comparison with experimental results demonstrates that the predictions of heat release rates are in reasonably good agreement with the experiment.     

Parameters ruling capillary forces at the submillimetric scale

This paper reviews the way to compute capillary forces between two solids by numerically integrating the Laplace equation describing the shape of an axially symmetric meniscus at equilibrium. The numerical results of the proposed model have been experimentally validated with a test bed able to measure forces of about 1 mN with an accuracy of about 1 microN. Thanks to the simulation tool and the test bed, the influence of the following parameters has been studied: surface tension, solid geometry, volume of liquid, materials, separation distance between both solids, and surrounding environment. The way to compute the force from a given meniscus geometry has been clarified as far as the "Laplace" and "tension" contributions are concerned.     

Recent advances in organic pressure-responsive luminescent materials

Pressure is a powerful tool to regulate the molecule aggregation and intermolecular interactions. Organic luminescent materials under high pressure can produce rich phenomena,which have many potential applications.     

Efficient simulation of binary adsorption isotherms using transition matrix Monte Carlo

Molecular simulations of binary adsorption in porous materials are a useful complement to experimental studies of mixture adsorption. Most molecular simulations of binary adsorption are performed using grand canonical Monte Carlo (GCMC) to independently examine a range of state points of interest. A disadvantage of this approach is that it only yields information at a discrete set of state points; therefore, if a complete isotherm is required for arbitrary conditions, some type of data fitting or interpolation must be used in combination with the GCMC data. We show that the transition matrix Monte Carlo (TMMC) method of Shen and Errington (Shen, V. K.; Errington, J. R. J. Chem.Phys. 2005, 122, 064508) is well-suited to simulation of binary adsorption in porous materials. At the completion of a TMMC simulation, the adsorption isotherm for all possible bulk phase compositions and pressures is available without data fitting or interpolation. It is also straightforward to use results from TMMC to compute derivatives of the isotherm such as the mixture thermodynamic correction factors, partial differential ln f(i)/partial differential ln c(j), again without data fitting or interpolation. This approach should be useful in contexts where information on the full adsorption isotherm is needed, such as the design of adsorption- or membrane-based separations.     

Phosphorus-based heteropentacenes: efficiently tunable materials for organic n-type semiconductors

Benzo-condensed dithieno[3,2-b:2',3'-d]phospholes have been synthesized that allow convenient tuning of properties that are essential for application as semiconductor materials in organic field-effect transistor (OFET) devices. The versatile reactivity of the trivalent phosphorus atom in these heteropentacenes provides access to a series of materials that show different photophysical properties, significantly different organization in the solid state, and distinctly different electrochemical properties that can be achieved by simple chemical modifications. The materials show strong photoluminescence in solution and in the solid state that depends on the electronic nature of the phosphorus center. Electrochemical studies revealed that the phosphorus atom intrinsically furnishes materials with n-channel or ambipolar behavior, also depending on its electronic nature. The experimental data were verified by DFT quantum chemical calculations and suggest that the phosphorus-based heteropentacenes could be excellent candidates for n-channel OFET semiconductor materials.