We investigate pairwise electrostatic interaction methods and show that there are viable computationally efficient (O(N)) alternatives to the Ewald summation for typical modern molecular simulations. These methods are extended from the damped and cutoff-neutralized Coulombic sum originally proposed by Wolf et al. [J. Chem. Phys. 110, 8255 (1999)]. One of these, the damped shifted force method, shows a remarkable ability to reproduce the energetic and dynamic characteristics exhibited by simulations employing lattice summation techniques. Comparisons were performed with this and other pairwise methods against the smooth particle-mesh Ewald summation to see how well they reproduce the energetics and dynamics of a variety of molecular simulations.

Using classical molecular dynamics simulations, we study ion-ion interactions in water. We study the potentials of mean force (PMF) for the full set of alkali halide ion pairs, and in each case, we test different parameter sets for modeling both the water and the ions. Altogether, we compared 300 different PMFs. We also calculate association equilibrium constants (K A ) and compare them to two types of experiments. Of additional interest here was the proposition of Collins called the 'law of matching water affinities', where the relative affinity of ions in solution depends on the matching of cation and anion sizes. From observations on the relative depths of the free energies of the contact ion pair (CIP) and the solvent-shared ion pair (SIP), along with related solvent structure analyses, we find a good correlation with this proposition: small-small and large-large should associate in water and small-large should be more dissociated.

How are water’s material properties encoded within the structure of the water molecule? This is pertinent to understanding Earth’s living systems, its materials, its geochemistry and geophysics, and a broad spectrum of its industrial chemistry. Water has distinctive liquid and solid properties: It is highly cohesive. It has volumetric anomalies—water’s solid (ice) floats on its liquid; pressure can melt the solid rather than freezing the liquid; heating can shrink the liquid. It has more solid phases than other materials. Its supercooled liquid has divergent thermodynamic response functions. Its glassy state is neither fragile nor strong. Its component ions—hydroxide and protons—diffuse much faster than other ions. Aqueous solvation of ions or oils entails large entropies and heat capacities. We review how these properties are encoded within water’s molecular structure and energies, as understood from theories, simulations, and experiments. Like simpler liquids, water molecules are nearly spherical and interact with each other through van der Waals forces. Unlike simpler liquids, water’s orientation-dependent hydrogen bonding leads to open tetrahedral cage-like structuring that contributes to its remarkable volumetric and thermal properties.

We study the solvation of polar molecules in water. The center of water's dipole moment is offset from its steric center. In common water models, the Lennard-Jones center is closer to the negatively charged oxygen than to the positively charged hydrogens. This asymmetry of water's charge sites leads to different hydration free energies of positive versus negative ions of the same size. Here, we explore these hydration effects for some hypothetical neutral solutes, and two real solutes, with molecular dynamics simulations using several different water models. We find that, like ions, polar solutes are solvated differently in water depending on the sign of the partial charges. Solutes having a large negative charge balancing diffuse positive charges are preferentially solvated relative to those having a large positive charge balancing diffuse negative charges. Asymmetries in hydration free energies can be as large as 10 kcal/mol for neutral benzene-sized solutes. These asymmetries are mainly enthalpic, arising primarily from the first solvation shell water structure. Such effects are not readily captured by implicit solvent models, which respond symmetrically with respect to charge.

We present a simple optimization strategy for incorporating experimental dielectric response information on neat liquids in classical molecular models of alcohol. Using this strategy, we determine simple and transferable hydroxyl modulation rules that, when applied to an existing molecular parameter set, result in a newly dielectric corrected (DC) parameter set. We applied these rules to the general Amber force field (GAFF) to form an initial set of GAFF-DC parameters, and we found this to lead to significant improvement in the calculated dielectric constant and hydration free energy values for a wide variety of small molecule alcohol models. Tests of the GAFF-DC parameters in the SAMPL4 blind prediction event for hydration show these changes improve agreement with experiment. Surprisingly, these simple modifications also outperform detailed quantum mechanical electric field calculations using a self-consistent reaction field environment coupling term. This work provides a potential benchmark for future developments in methods for representing condensed-phase environments in electronic structure calculations.

Molecular simulations often use explicit-solvent models. Sometimes explicit-solvent models can give inaccurate values for basic liquid properties, such as the density, heat capacity, and permittivity, as well as inaccurate values for molecular transfer free energies. Such errors have motivated the development of more complex solvents, such as polarizable models. We describe an alternative here. We give new fixed-charge models of solvents for molecular simulations – water, carbon tetrachloride, chloroform and dichloromethane. Normally, such solvent models are parameterized to agree with experimental values of the neat liquid density and enthalpy of vaporization. Here, in addition to those properties, our parameters are chosen to give the correct dielectric constant. We find that these new parameterizations also happen to give better values for other properties, such as the self-diffusion coefficient. We believe that parameterizing fixed-charge solvent models to fit experimental dielectric constants may provide better and more efficient ways to treat solvents in computer simulations.

We performed a combined calorimetric and molecular modeling investigation of poly(vinyl acetate) (PVAc) on silica to characterize the intermolecular interactions and the behavior of the adsorbed polymer. From temperature-modulated differential scanning calorimetry experiments, different regions of thermal activity suggested a gradient of mobility in the adsorbed polymer. Polymer segments in more direct contact with silica (tightly bound) showed a significantly elevated and broadened glass transition relative to the bulk polymer, while polymer further away (loosely bound) showed only a slightly elevated transition relative to the bulk polymer. A thermal transition for PVAc at the air interface (more-mobile) was also observed and was at lower temperatures than the bulk polymer. Density profiles from molecular dynamics studies suggested a structure of the adsorbed polymer similar to that experimentally observed. These studies were consistent with the presence of a motional gradient in the polymer segments, and concomitant glass transition changes from the silica to the air interfaces. These results also demonstrate that hydrogen-bonding interactions, at the PVAc/silica interface, are critical to the high-temperature shifts in the glass transition. ■ INTRODUCTIONAdsorbed polymer−substrate interactions usually lead to differences in properties of bulk and adsorbed polymers. 1−10 Interactions between adsorbed polymers and solid surfaces have been shown to provide advantageous physical, mechanical, and thermal properties, making these materials suitable as lubricants, adhesives, coatings, and corrosion-resistant agents. 11−17 These properties are closely related to those that determine the glass transition, which for small amounts of adsorbed polymers depend on film thickness, polymer molecular mass, intermolecular interactions, and the mobility of macromolecular chains. 18−20 For example, the T g will be elevated if the interactions between the polymers and the substrate are attractive and strong. 21 Strong attractive interactions, covalent or hydrogen bonding, between polymer segments and the substrate can potentially reduce the mobility of the adsorbed polymer segments. This reduction in mobility due to restrictions from attachment points has been proposed as the main reason for T g elevation. 22−24 Differential scanning calorimetry (DSC) is the most common technique used to investigate thermal characteristics of bulk polymers and composites. 25−27 Temperature-modulated DSC (TMDSC) is a variant of DSC that, in addition to providing the same information as conventional DSC, provides additional insight into the thermal behavior of materials by separating the heat flow data into reversing and nonreversing events. 28,29 TMDSC and its derivatives have been used to resolve both weak and multicomponent transitions that would be difficult to distinguish in a conventional DSC scan. 30−34 Molecular dynamics (MD) simulations have also been used to investigate the dynamics and thermodynamics of thin-film polymer coatings. 35−5...

We describe a computational solvation model called semi-explicit assembly (SEA). SEA water captures much of the physics of explicitsolvent models but with computational speeds approaching those of implicit-solvent models. We use an explicit-water model to precompute properties of water solvation shells around simple spheres, then assemble a solute's solvation shell by combining the shells of these spheres. SEA improves upon implicit-solvent models of solvation free energies by accounting for local solute curvature, accounting for near-neighbor nonadditivities, and treating water's dipole as being asymmetrical with respect to positive or negative solute charges. SEA does not involve parameter fitting, because parameters come from the given underlying explicitsolvation model. SEA is about as accurate as explicit simulations as shown by comparisons against four different homologous alkyl series, a set of 504 varied solutes, solutes taken retrospectively from two solvation-prediction events, and a hypothetical polarsolute series, and SEA is about 100-fold faster than Poisson-Boltzmann calculations.free energy | implicit solvent | transfer W e describe here an approach for computing the free energies of solvation of solutes in water. Aqueous solvation has been modeled at different levels, ranging from detailed quantum mechanics simulations of few-molecule clusters (1, 2), to faster classical simulations using up to tens of thousands of explicit molecules (3-10), to very fast models in which water is treated implicitly as a simple uniform continuous medium (11)(12)(13)(14)(15)(16)(17). For large computations, such as those in typical biomolecule simulations, explicit-water modeling can be slow and expensive, so it is common to use implicit water instead. However, implicit models often require trade-offs in the physics that can limit their accuracies. For example, water is typically treated as a continuum rather than individual particles, and this neglects discrete microscopic effects; nonpolar solvation effects are often assumed to depend only on surface area A (expressed as γA), and not on detailed dispersive interactions and collective consequences of solute shape (18)(19)(20).It would be useful to have a computational model of water that is both fast-approaching the speeds of the fastest implicitsolvent models-and that captures the physics and the transferability of explicit-solvent models. Toward this goal, various improvements of implicit models have been introduced (21, 22), explicit solvents have been coarse-grained (23, 24), and hybrid explicit-implicit models have been developed (25)(26)(27)(28)(29). Here, we take a different approach. We precompute solvation properties of water in explicit-solvent simulations of simple spheres, which we then apply in summations over assemblies about arbitrary solutes. As the details of the solvation response come entirely from the physics of an explicit solvent, this model lacks free parameters from statistical fits to solute molecular transfer free energies, resulting in a ...