  Structural and thermodynamic properties of high-density amorphous (HDA) ice
have been studied by path-integral molecular dynamics simulations in the
isothermal-isobaric ensemble. Interatomic interactions were modeled by using
the effective q-TIP4P/F potential for flexible water. Quantum nuclear motion is
found to affect several observable properties of the amorphous solid. At low
temperature (T = 50 K) the molar volume of HDA ice is found to increase by 6%,
and the intramolecular O--H distance rises by 1.4% due to quantum motion. Peaks
in the radial distribution function of HDA ice are broadened respect to their
classical expectancy. The bulk modulus, B, is found to rise linearly with the
pressure, with a slope dB/dP = 7.1. Our results are compared with those derived
earlier from classical and path-integral simulations of HDA ice. We discuss
similarities and discrepancies with those earlier simulations.
