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This paper deals with the preparation of novel polytetrafluoroethylene (PTFE) membranes from an aqueous dispersion of fluorinated polymer and a pore forming agent (PFA) sintered onto the outer surface of a porous tubular support. Different membranes were obtained by varying the ratio between PTFE and PFA in the starting dispersion as well as the number of dispersion layers sintered on the support. Membranes were characterized through scanning electron microscopy and gas–liquid displacement porosimetry. Distillation tests of the membrane were carried out on a laboratory scale unit fed with a NaCl solution. The characteristics and performance of the membranes were compared with those of commercial porous PTFE tubulets.

The improvement in the convergence rate of the damped multipole expansion for the second order H-H+ induction energy due to the inclusion into the first-order function of a term satisfying the electron-proton cusp condition is quantitatively investigated. Following a technique suggested by Schwartz, the multipole contributions to the energy are put in the form of series ordered by inverse powers of the quantum number l. It is shown that, for high values of l, the dependence of the leading terms is raised from l -4 to l -6 by the cusp correction.

Following early work by Kutzelnigg on the correlation cusp, we have attempted to improve convergence of the damped multipole expansion of the second-order induction energy for H+ + by including a separately determined cusp correction of the form φ = f(r B)ψ0 into the first-order function in order to avoid divergency at the proton position when r B = 0. Three different cusp wavefunctions, all derived from the Taylor expansion of f(r B) into positive powers of r B have been examined, and the best compromise between simplicity and accuracy is found for the function φ = -r Bψ0. Few residual multipoles, variationally approximated in terms of one-centre linear pseudostates, are then found necessary to reproduce accurately the induction energy at the distance of the chemical bond.

The N-term representation of the H atom static propagator in terms of a finite discrete set of linear excited pseudostates involving positive powers of the radial variable is used to calculate multipole components of nonexpanded induction and dispersion interactions at the bond length for H+ 2 and H2, nonexpanded dispersion coefficients for H2, and hydrogenic second-order energy for the He sequence. Even if convergence is sensibly slower than that observed for expanded perturbations, the N = 20 approximation is found to give results at worst within 1% of the accurate reference values from the literature.

The perturbative configuration interaction approach based on non-orthogonal bond-orbitals previously used for dealing with rotational barriers is applied to the study of the hydrogen bonding in the linear water dimer. First and second-order interaction energies are obtained in terms of static and transition charge distributions fully accounting for intermolecular overlap. Neglecting electron correlation, the second-order calculations include all single excitations from bonding to antibonding orbitals accounting for induction including exchange and giving results close to the corresponding supermolecular SCF-MOs in the same basis. Ab initio calculations using different gaussian minimal bases show that Clementi's GTO basis MEDIUM is the most suitable for describing molecular interactions. Detailed component analysis of the energy up to second order is possible and reveals the main features of the intermolecular hydrogen bonding occurring between the water molecules.