  Classical and path integral molecular dynamics (PIMD) simulations are used to
study alpha-quartz and beta-quartz in a large range of temperatures at zero
external stress. PIMD account for quantum fluctuations of atomic vibrations,
which can modify material properties at temperatures below the Debye
temperature. The difference between classical and quantum mechanical results
for bond lengths, bond angles, elastic modulii, and some dynamical properties
is calculated and comparison to experimental data is done. Only quantum
mechanical simulations are able to reflect the correct thermomechanical
properties below room temperature. It is discussed in how far classical and
PIMD simulations can be helpful in constructing improved potential energy
surfaces for silica.
