A general and efficient methodology is presented which allows molecules containing one or many rings of any size to be manipulated within energy minimization procedures. Variables describing the conformation of the molecules concerned are limited to dihedral and ring valence angles and the ring closure conditions are treated as equality constraints. An application is made to the ion transporter valinomycin and its complexes with K+ and Na+ which illustrates the possibilities of the approach and leads to results which allow a better understanding of the conformational mechanics of this important ionophore.

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The authors report on transport and optical measurements of some alpha '-phase (BEDT-TTF)2X salts with X=AuBr2-, Ag(CN)2- and CuCl2-. These salts are shown to be Mott-Hubbard semiconductors. Transport measurements were made under hydrostatic pressures up to 20 kbar and temperatures down to 4 K. At room temperature all three systems have low conductivities, approximately=0.05 S cm-1 and small activation energies, approximately=300 meV. On cooling, the AuBr2- and Ag(CN)2- salts show a transition between two semiconducting phases with slightly different activation energies, and for both salts the transition temperature increases under pressure. For all three salts the activation energy for conductivity decreases under pressure. In the AuBr2- system the transition changes abruptly from second to first order at 8.5 kbar. This is not seen for Ag(CN)2-. The thermopower has been measured for all three salts at ambient pressure. There are differences, with the AuBr2- system showing hole conduction and the other showing electron conduction. The authors have measured the optical absorption spectrum of the Ag(CN)2- salt, and this shows a peak at 1.3 eV in the solid state which is missing in a dilute solution of BEDT-TTF. They attribute this to a Hubbard correlation. Finally, they suggest an explanation for the similarities of the differences in behaviour between these three systems and discuss the general properties in terms of a large-U model.

The discovery of a low-temperature superconducting state in organic compounds of the type (TMTSF)CIO (T = 1.2 K) and (BEDT-TTF)AuI (T = 4 K) (where TMTSF is tetramethyltetraselenafulvalene, BEDT-TTF is bis(ethylenedithiolo)tetrathiafalvalene and T is the superconducting transition temperature) has stimulated the search for new materials that may show higher values of T (refs 1-3). The general problem encountered in molecular charge-transfer salts of this type, which have conduction bands formed by intermolecular overlap of π-electron systems, is that conduction is usually quasi-one-dimensional, with good conduction along the stacking direction. Metals with this one-dimensional character are unstable, and undergo a Peierls transition to a semiconducting state at low temperatures. The relatively few exceptions (mentioned above), which remain metallic down to low temperatures, are considered to do so because they show stronger interstack interactions. We report here a new material with inherently two-dimensional interactions between the molecular π-electron systems and which we are able to stabilize as a metal down to low temperatures (1.4 K) under hydrostatic pressure (12 kbar).

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