  We report ab-inito density functional theory calculation of phonons in cubic
phase of ZrW2O8 in the entire Brillouin zone and identify specific anharmonic
phonons that are responsible for large negative thermal expansion (NTE) in
terms of translation, rotation and distortion of WO4 and ZrO6. We have used
density functional calculations to interpret the experimental phonon spectra as
a function of pressure and temperature as reported in literature. We discover
that the phonons showing anharmonicty with temperature are different from those
showing anharmonicity with pressure although both are of similar frequencies.
Only the latter phonons are associated with NTE. Therefore the cubic and/or
quadratic anharmonicity of phonons is not relevant to NTE but just the volume
dependence of frequencies. The calculations are able to reproduce the observed
anomalous trends, namely, the softening of the low frequency peak at about 5
meV in the phonon spectra with pressure and its hardening with temperature,
while both the changes involve a compression of the lattice.
