# Manual:External utilities:oct-vibrational spectrum

This utility calculates the vibrational spectrum from a molecular-dynamics run.

What this utility does is to read the velocity from the td.general/coordinates and calculate the Velocity Autocorrelation Function:

$C_{v}(t)=\sum_{i=1}^{N_{atoms}}\vec{v}_i(t)\cdot\vec{v}_i(t_0)\ ,$

afterward a cosinusoidal envelope is added, to make the periodic extension of the function continuous and then the spectrum is calculated by taking the Fourier transform of the function. On exit, two files are generated td.general/velocity_autocorrelation and td.general/vibrational.

This utility honours the variables PropagationSpectrumStartTime and PropagationSpectrumEndTime to control the time of sampling. Note that the velocity in the initial time must be different from zero, or $C_{v}$ will be identically zero.

As a discrete Fourier tranform is used, this utility can take several minutes to process a large run. If Octopus was compiled with OpenMP support, this utility can run in several threads.

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