The static dipole polarizabilities of helium and molecular hydrogen by differential diffusion Monte Carlo

The differential diffusion Monte Carlo method, involving correlated random walks, is used to calculate the static polarizabilities of molecular hydrogen and helium by application of a finite electrostatic field. The results are for molecular hydrogen (alpha)=4.60(3) au; (alpha)|=6.38(5) au; for helium (alpha) = 1.38(1) au. The results agree, within the statistical errors, with those obtained by application of high quality ab initio methods

Irrelevance of atomic masses for Debye-Waller B values in the limit of high temperatures

An expression that approximates Debye-Waller B values by a sum of three terms is derived from the theory of lattice dynamics in the harmonic approximation. For cubic crystals (M is the mass of the th atom in the unit cell): B = T + 22h2/3k TM + /M2T3, where T D/2 and and are constants, depending on interatomic forces only. It is shown that for temperatures above the Debye temperature D of the lattice, the second and third terms in the above expression can be neglected. From this, it follows that above the Debye temperature Debye-Waller B values become independent of the atomic masses. Consequently, the heavier atoms in a lattice do not necessarily have the smaller B values

Debye-Waller B values for some NaCl-type structures and interionic interaction

Debye-Waller B values of NaCl, KCl, MgO and AgCl are calculated from lattice dynamics based on a simple, rigid-ion type, interaction model. The parameters introduced in the model have intelligible significance. The present approach compares favourably with experimental evidence and previous physical calculations

Unstable vortices do not confine

Recently, a geometric model for the confinement of magnetic charges in the context of type II string compactifications was constructed by Greene, Morrison and Vafa. This model assumes the existence of stable magnetic vortices with quantized flux in the low energy theory. However, quantization of flux alone does not imply that the vortex is stable, since the flux may not be confined to a tube of definite size. We show that in the field theoretical model which underlies the geometric model of confinement, static, cylindrically symmetric magnetic vortices do not exist. While our results do not preclude the existence of confinement in a different low-energy regime of string theory, they show that confinement is not a universal outcome of the string picture, and its origin in the low energy theory remains to be understood.Comment: Latex, 8 page

Crosscaps, Boundaries and T-Duality

Open descendants with boundaries and crosscaps of non-trivial automorphism type are studied. We focus on the case where the bulk symmetry is broken to a Z_2 orbifold subalgebra. By requiring positivity and integrality for the open sector, we derive a unique crosscap of automorphism type g \in Z_2 and a corresponding g-twisted Klein bottle for a charge conjugation invariant. As a specific example, we use T-duality to construct the descendants of the true diagonal invariant with symmetry preserving crosscaps and boundaries.Comment: Latex, 14 page

The crystal and molecular structure of 2,7-diazanaphthalene

X-ray diffraction data were collected at 20°C on a computer-controlled Philips diffractometer (PW 1100). The structure was solved by direct methods and refined by the full-matrix least-squares method to an R of 0.065 (weighted R 0.046). The molecule in the crystal is of lower symmetry than C2,.. The geometry of the molecule has been compared with NMR results obtained by Danieli, Lunazzi & Veracini [J. Chem. Soc. Perkin II, (1976), pp. 19-201. As a result a C-H bond-length correction of 0.09 (2) A was found