Effect of basis-set completeness on the relative importance of triply and quadruply excited configurations in correlation energy calculations

It is demonstrated, using the ground state of the water molecule, that the contribution of triply excited configurations to electron correlation energies in closed-shell systems is more sensitive to the degree of completeness of the basis set than the component associated with quadruply excited configurations. The relative importance of the triply excited states increases more rapidly than that of the quadruple excitations, as the quality of the basis set is improved.     

Stability experiments on the strongest columns and circular arches

Recent theories predict the cross-sectional shapes for the strongest, end-loaded column and the radially loaded arch whose neutral axis is circular. These tapered shapes have higher predicted load-carrying capacities than uniform columns or arches of equal weight and length or span. The purpose of the present experimental investigation was to study the stability of these configurations, modified so that no experimental model had the predicted cross-sectional area of zero where the bending moments vanished. Precise buckling loads were measured on metal models of tapered, pinned-end columns using strain gages and a unique modification of Southwell's method. The dynamic stability of these end-loaded columns under transverse vibrations was observed. Static stability tests were also performed on Plexiglas models of the strongest shaped circular arch and on the uniform circular arch of equal weight and span. The predicted buckling loads for the strongest shapes agreed reasonably well with measurements. The shaped circular arch, however, was found to be an inefficient design for loading applied uniformly across the span rather than in the radial direction.     

Adiabatic compressibility of polymer solutions—II. Polystyrene in toluene

Adiabatic compressibility data for polystyrene, using samples of various molecular weights dissolved in toluene, are reported as a function of concentration. These data are compared with calculations of the incremental compressibility obtained from viscoelastic and longitudinal acoustic relaxation studies. The differences between the observed and calculated increments are attributed to polymer-solvent interactions.