Vibration spectra from velocity ACF or Normal modes.

Vibration spectra from velocity ACF or Normal modes.

The velocity autocorrelation function can be converted into
a vibrational spectrum by performing a fourier transform
and plotting the amplitude squared as a function of
frequency or wavenumber. This has now been automated in
g_velacc with the -os flag. Likewise, an option -os
has been added to the g_nmeig program to produce a spectrum
based on the harmonic approximation.

Change-Id: I7e3d669b9108e3a91856ff20688f6216a035bf4a