Implications of rotation–inversion–permutation invariance for analytic molecular potential energy surfaces

A molecular potential energy surface has the symmetry properties of invariance to rotation of the whole molecule, inversion of all atomic coordinates, and permutation of indistinguishable nuclei. While some of this invariance character can be easily incorporated in a local description of the surface, a formal application of these symmetry restrictions is useful in considering the form of the globalsurface which must account for large amplitude changes of the atomic coordinates. The form of a global molecular potential energy surface as a properly symmetrized analytic function of Cartesian coordinates is derived by extending Molien’s theorem of invariants for finite groups to cover the continuous rotation–inversion group. O(3), and the product of O(3) with the complete nuclear permutation group. The role of so‐called redundant internal coordinates in molecular potential energy surfaces is clarified

Nonlocal continuum solvation model with oscillating susceptibility kernels: A nonrigid cavity model

A nonlocal continuum theory of solvation is applied using an oscillatingdielectric function with spatial dispersion. It is found that a convergent solution cannot be calculated using a model of a fixed solute cavity inside the solvent continuum. This is attributed to the fact that the dielectricoscillations appear as a result of coupling between polarization and density fluctuations, contradicting the concept of a fixed cavity. The theory is corrected by allowing the cavity size to vary. A cavitation energy and an interaction between the medium reaction field and the cavity size are added to the solvation free energy, and a new theory obtained by a variational treatment. The interaction term enables convergent solutions to become attainable, resulting in an oscillatingelectrostatic solvation energy as a function of cavity radius, the cavitation term enables these oscillations to be smoothed out, resulting in a regular, monotonic solvation free energy

Globule transitions of a single homopolymer: A Wang-Landau Monte Carlo study

The temperature-independent Wang-Landau Monte Carlo approach is implemented for an off-lattice model of flexible homopolymers and applied to the coil-globule and solidification transitions based on chain sizes up to N=300. An intermediate transformation from low-density liquid globule to high-density liquid globule is suggested. A scheme for identifying polymer structures representative of particular temperatures in the course of the simulation is presented and applied to illustrate intermediate states in the coil-globule transition. Transition temperatures are calculated and used to obtain a theta point of at least Θ=1.96, distinctly higher than the solid-liquid transition temperature TM =1.26

Erratum: “An advanced dielectric continuum approach for treating solvation effects: Time correlation functions. I. Local treatment” [J. Chem. Phys. 108, 1103 (1998)]

A local continuum solvation theory, exactly treating electrostatic matching conditions on the boundary of a cavity occupied by a solute particle, is extended to cover time-dependent solvation phenomena. The corresponding integral equation is solved with a complex-valued frequency-dependent dielectric function ε(ω), resulting in a complex-valued ω-dependent reaction field. The inverse Fourier transform then produces the real-valued solvation energy, presented in the form of a time correlation function (TCF). We applied this technique to describe the solvation TCF for a benzophenone anion in Debye (acetonitrile) and two-mode Debye (dimethylformamide) solvents. For the Debye solvent the TCF is described by two exponential components, for the two-mode Debye solvent, by three. The overall dynamics in each case is longer than that given by the simple continuum model. We also consider a steady-state kinetic regime and the corresponding rate constant for adiabatic electron-transferreactions. Here the boundary effect introduced within a frequency-dependent theory generates only a small effect in comparison with calculations made within the static continuum model

Surface forces: Surface roughness in theory and experiment

A method of incorporating surface roughness into theoretical calculations of surface forces is presented. The model contains two chief elements. First, surface roughness is represented as a probability distribution of surface heights around an average surface height. A roughness-averaged force is determined by taking an average of the classic flat-surface force, weighing all possible separation distances against the probability distributions of surface heights. Second the model adds a repulsive contact force due to the elastic contact of asperities. We derive a simple analytic expression for the contact force. The general impact of roughness is to amplify the long range behaviour of noncontact (DLVO) forces. The impact of the elastic contact force is to provide a repulsive wall which is felt at a separation between surfaces that scales with the root-mean-square (RMS) roughness of the surfaces. The model therefore provides a means of distinguishing between "true zero," where the separation between the average centres of each surface is zero, and "apparent zero," defined by the onset of the repulsive contact wall. A normal distribution may be assumed for the surface probability distribution, characterised by the RMS roughness measured by atomic force microscopy (AFM). Alternatively the probability distribution may be defined by the histogram of heights measured by AFM. Both methods of treating surface roughness are compared against the classic smooth surface calculation and experimental AFM measurement

Sickness experience and language : aspects of Tongan and Western accounting

In this study of Tongan healing practices, the author has chosen not simply to record participants’ roles or medicinal preparations but rather, the concern of this study is to understand the ‘doing’ of sickness as a social practice. Two main sociological techniques have been applied. Firstly, a hermeneutic - phenomenological approach was used to attain recordings of, and to analyze, the sickness theorizing of members of Tongan society. The ‘sickness talk’ of these members provides a record of some aspects of contemporary Tongan healing practices. The ‘sickness talk’ is also analyzed using Wittgenstein’s concept of ‘rule-usage’ in the ‘language game’ of sickness. This form of analysis indicated certain individual and public relevances which are grounded upon the Tongan way of life. Thus speakers’ accounts are analyzed in terms of what sickness talk ‘shows’ as well as what it ‘says’, disclosing cultural process instead of simply cultural product. Discussion on ‘diagnosis’ shows that in order to define the problematic situation of sickness, the phenomena are organized into a classification, that is, members have to ‘capture’, ‘fix’, ‘concretize’ the confronting transient phenomena and apply a sickness label as a ‘working definition’. Tongan sickness ‘types’ are shown to be not only different from Western ‘types’, but aspects of the process of constructing that difference are also shown to be implicit in the sickness talk. Diagnosis as a social process is not seen as the labelling of an ‘objective fact’, ‘a sickness’; nor is therapy understood as being some ‘thing’ that gets a person ‘better’. Tongan explanatory models, developed to explain sickness causation, differ essentially from Western explanatory models of sickness in that Tongans have developed a social model of prevention and cure while in the West, a biological model has been developed. Explanation is understood here not as the causal accounting of sickness but as the explanation of enigmatic consequences. The latter sections of the study on doing sickness as ‘kinship’ and as ‘healer’ not only add to the record on contemporary healing practices but also emphasize the Tongan social model of sickness. This study therefore, is not only a record, albeit a partial one, of contemporary Tongan healing practices, it also shows how these particular Tongans define certain sickness situations and devise a strategy to resolve that problematic situation. That is, it shows how committing certain experiences to language ‘is’ the ordering process. Rather than any magico-religious or scientific-biological model providing the basis of sickness practice in Tongan society, kinship is proposed as the underlying organizing principle. The comparative mode of analysis, in relation to Tongan and Western sickness theorizing, avoids presenting Western explanation as a model by which Tongan theorizing can be evaluated. Instead, in analyzing Tongan healing practices showing knowledge and relevance(s), substantive dimensions of Western theorizing and practice are disclosed. In selecting a limited number of members’ sickness accounts over a period of six and a half months in Tonga, I have not attempted to randomly sample the Tongan population in order to generalize my ‘findings’ to the whole of Tongan society. Instead my interest has been to give an interpretation of some aspects of Tongan sickness theorizing which may or may not be altered by similar and more extensive studies in the future. Contrary to what may be seen as being medical and anthropological expectations, Tongan traditional healing practices have not declined since Western contact, rather, they have developed from reportedly limited skills at that time, to an extensive network of healing practice today

Surface forces in particle technology: Wet systems

Surface forces play a fundamental role in particle processing as they control the stability, adhesion, friction and rheology of particulate systems and information on all of these can be obtained from an analysis of the normal forces measured between particles. Therefore particle processing at all stages can be informed by knowledge of the forces between the constituent particles. For wet particles systems, the interaction forces between two particles can rarely be predicted from theory, but rather requires experimentation or direct measurement. This requires that the surfaces used have the same as surface properties as the particles. In practice this is rarely possible, as surface force measurements require surfaces with extremely low roughness and precise geometry and the majority of materials do not conform to these requirements. To address these challenges we produce surfaces of low roughness and controlled chemistry using Atomic Layer Deposition (ALD) and are developing methods to calculate and understand the influence of surface roughness on the measured forces. Here we report the forces between hafnia surfaces produced by ALD and show that like ALD produced titania surfaces and silica surfaces, the expected van der Waals forces at high pH are not manifest, suggesting that most real surfaces have unexpectedly repulsive surface forces at high pH and small separations. This will fundamentally alter how these particulate systems behave when being processed, reducing the adhesion and the friction and enhancing the stability compared to the expected interaction from DLVO theory

Increased porosity turns desorption to adsorption for gas bubbles near water-SiO2 interface

We consider theoretically the retarded van der Waals interaction of a small gas bubble in water with a porous SiO2 surface. We predict a possible transition from repulsion to attraction as the surface is made more porous. It highlights that bubbles will interact differently with surface regions with different porosity (i.e., with different optical properties)

Fluid-sensitive nanoscale switching with quantum levitation controlled by α -Sn/ β -Sn phase transition

We analyze the Lifshitz pressure between silica and tin separated by a liquid mixture of bromobenzene and chlorobenzene. We show that the phase transition from semimetallic α−Sn to metallic β−Sn can switch Lifshitz forces from repulsive to attractive. This effect is caused by the difference in dielectric functions of α−Sn and β−Sn, giving both attractive and repulsive contributions to the total Lifshitz pressure in different frequency regions controlled by the composition of the intervening liquid mixture. In this way, one may be able to produce phase-transition-controlled quantum levitation in a liquid medium