Vapour—liquid equilibrium of the square-well fluid of variable range via a hybrid simulation approach

The equilibrium between vapour and liquid in a square-well system has been determined by a hybrid simulation approach combining chemical potentials calculated via the Gibbs ensemble Monte Carlo technique with pressures calculated by the standard NVT Monte Carlo method. The phase equilibrium was determined from the thermodynamic conditions of equality of pressure and chemical potential between the two phases. The results of this hybrid approach were tested by independent NPT and μPT calculations and are shown to be of much higher accuracy than those of conventional GEMC simulations. The coexistence curves, vapour pressures and critical points were determined for SW systems of interaction ranges λ = 1.25, 1.5, 1.75 and 2. The new results show a systematic dependence on the range λ, in agreement with results from perturbation theory where previous work had shown more erratic behaviour.     

A model for viscosity coefficients of gases with potentials differing in form

A recent theory of non-conformal interactions has been very succesful in providing effective spherical potentials for the pressure of more than 40 real gases and many of their binary mixtures. Here, this theory is applied to deal with low-density viscosity coefficients. In its simplest form, the approximate non-conformal (ANC) theory introduces, besides the usual corresponding states parameters-energy ? and distance r_m-a softness parameter s to account for the form of a particular potential function. We investigate the effects of the potential form on the temperature behaviour of the viscosisty coefficient η. It is shown that the softer potentials, with wider attractive wells, have larger viscosities and an explicit expression for η (T, ?, r_m, s) is obtained. The ANC potentials are tested in their capacity to reproduce the temperature dependence of η for the heavier noble gases (Ar, Kr and Xe), diatomics (H₂, N₂, O₂ and CI₂) and a dozen small polyatomics. It is found that the ANC model η (T, ?, r_m, s), with only three substance-dependent parameters, reproduces experimental 7 data within their estimated error.     

What is so mysterious about the electronic states of SCI?

The 15 lowest lying doublet electronic states of the molecule SCI have been investigated theoretically at a high level of correlation treatment (MRCI). For the ground state (X ²II), spectroscopic constants were obtained from a set of eight vibrational intervals. This result extends our knowledge about this state beyond the experimentally known data that presently were derived from only two bands. Spin-orbit constants, transition probabilities and radiative lifetimes complement its spectroscopic characterization. For the excited electronic states, a global view of the doublet states is presented that can help us understand the scarcity of experimental data on electronic transitions for this system and also the difficulty of assigning the only two transitions so far recorded. Most of these states are repulsive, and for the few high lying bound ones, of Rydberg character, avoided crossings restrict the number of accessible vibrational states. Crossing by repulsive states and predissociation is also a factor that can prevent further emissions. Two new bound excited states, ²δ and ²Σ, predicted in this study, are of direct relevance to an interpretation of the limited experimental data available on electronic transitions.     

A Family of Examples with Quantum Constraints

In Rep. Math. Phys. 35 (1995), 101, the authors describe a method for constructing directly (i.e. without using explicitly any field operator nor any concrete representation of the C*-algebra) nets of local C*-algebras associated to massless models with arbitrary helicity and that satisfy Haag–Kastler's axioms. In order to specify the sesquilinear and the symplectic form of the CAR- and CCR-algebras, respectively, a certain operator-valued function is introduced. This function is shown to be very useful in proving the covariance and causality of the net and it also codes the degenerate character of massless models with respect to massive models.It is the intention of this Letter to point out that the massless bosonic examples with helicity bigger than 0 fit completely into the general theory that Grundling and Hurst used to describe systems with gauge degeneracy.