  The structure of random sphere packings in mechanical equilibrium in
prescribed stress states, as studied by molecular dynamics simulations,
strongly depends on the assembling procedure. Frictionless packings in the
limit of low pressure are devoid of dilatancy, and consequently share the same
random close packing density, but exhibit fabric anisotropy related to stress
anisotropy. Efficient compaction methods can be viewed as routes to circumvent
the influence of friction. Simulations designed to resemble two such
procedures, lubrication and vibration (or ``tapping'') show that the resulting
granular structures differ, the less dense one having, remarkably, the larger
coordination number. Density, coordination number and fabric can thus vary
independently. Calculations of elastic moduli and comparisons with experimental
results suggest that measurable elastic properties provide information on those
important internal state variables.
