  Semi-empirical quantum chemistry methods offer a very interesting compromise
between accuracy and computational load. In order to assess the performance of
NDDO methods in the interpretation of terahertz spectra, the low frequency
vibration modes of three crystalline materials, namely, polyethylene,
poly(vinylidene fluoride) form II and $\alpha$-D-glucose have been studied
using the PM6 and PM7 Hamiltonians and the results have been compared with the
experimental data and former calculations. The results show good qualitative or
semi-quantitative agreement with the experimentally observed terahertz spectra.
