A Stefan-Type Problem with Void Formation and its Explicit Solution

For most materials, the densities of the solid and liquid phasesare different. For some materials being considered as potentiallatent heat thermal energy storage materials, this density changeis large. When the solid phase is more dense than the liquid,solidification may result in the formation of vapour bubblesor voids. We have formulated a problem, representing a phasechange process with such a void, that has an explicit solution.We examine the solution to gain a further understanding of thethermal and phase change performance of a material in whichsuch voids form. We use the solution to analyse an example probleminvolving freezing lithium fluoride, a candidate phase-changematerial with a density change on freezing of about 25 per cent.     

Monte Carlo simulations of an amphiphilic polymer at a hydrophobic/hydrophilic interface

Monte Carlo simulations have been carried out for an off-lattice model of an amphiphilic polymer at a hydrophobic/hydrophilic interface. The model system consists of a polynorbornene backbone with poly(ethylene oxide) (PEO) grafts modelled atomistically at an idealized interface between hydrophobic and hydrophilic regions, which are represented by external potentials. Results are presented for the distribution of PEO chain ends, and the density of PEO segments perpendicular to the surface. The latter is used to provide predictions for neutron reflectivity profiles normal to the surface as a function of the lateral confinement of the PEO grafts. At low surface coverage the simulation results are found to be in good agreement with experimental neutron scattering results from similar polymers studied at the water/air interface.     

The singlet electronic states of pyrrole: a theoretical study by both ab initio multi-reference configuration interaction methods and time-dependent density functional theory and a reconsideration of the experimental VUV spectral data

The singlet electronic excitation spectrum of pyrrole has been reinvestigated by both multi-reference multi-root configuration interaction (CI) calculations and time-dependent density functional theory (DFT) with asymptotically corrected exchange-correlation potentials. The methods used a triple zeta valence + polarization + Rydberg (TZVPR) basis set and a much larger active space than in our previous CI study [Palmer, M. H., Walker, I. C., and Guest, M. F., 1998, Chem. Phys., 238, 179]. Computed vertical excitation energies, oscillator strengths and electronic charge distributions were used to characterize and assign the valence and Rydberg excited states over an energy range of 5–12 eV.

A comparison of the present methods with other high-level ab initio studies has been made, including the effects of basis sets and size of CI, and some statistical relationships determined. The present CI vertical excitation energies are generally in closer agreement with the cluster-type methods, especially CC3, than to the various second-order perturbation-type methods (CASPT2, CASPT2-MS, ADC(2) and MRMP).

The influence on the excitation energies from exact orbital exchange and multiplicative potentials in hybrid functional development has been investigated. Differences between the CI and the DFT methods vary in the order B97-2 < B97-1 < HCTH < LDA. The differences between hybrid DFT and CI excitations are minimized when the fraction of orbital exchange (ξ) lies in the approximate range 0.2–0.3. The Rydberg and valence-type excitations are seen to be less sensitive than the static polarizability to the inclusion of orbital exchange or multiplicative potentials in hybrid functional development.

In order to allow a realistic assessment of the performance of the theoretical studies, the assignment of the experimental electronic spectrum of pyrrole is discussed in detail. Previous conclusions have led to incorrect numbers of Rydberg s- and d-type states, while f-type states have previously been ignored. Some excitations from the second IP, which must occur in the 5–10 eV range, have been reassigned in light of the known small differences between other spectroscopic states and quantum defects. There is an urgent need for higher-resolution studies of pyrrole and related molecules.     

On the use of MP2 theory for electron molecules correlation in atoms and molecules

The use of Moeller-Plesset perturbation theory, frequently designated MPn, in describing electron correlation in atomic and molecular systems is critically examined. Some recently published studies have examined the higher terms in the Moeller-Plesset perturbation expansion and the convergence behaviour of the perturbation series. From these investigations conclusions about the applicability of the low order theory have been drawn which, in view of the widespread use of MP2, demand more thorough analysis. Combining the Rayleigh-Schrödinger and the Brillouin-Wigner perturbation expansions avoids an infinite order expansion and yields a closed expression which consists of the MP2 energy components together with a remainder term. The applicablity of MP2 theory then rests upon the magnitude of this remainder term rather than the behaviour of the higher order terms in the perturbation series.