md_trajectory = nlread('alumina_trajectory.nc')[-1]

# Calculate the velocity autocorrelation function of the oxygen atoms.
vacf = VelocityAutocorrelation(md_trajectory, atom_selection=Oxygen)

# Get the time and autocorrelation values.
times     = vacf.times().inUnitsOf(fs)
vacf_data = vacf.data()

# Plot the data using pylab.
import pylab

pylab.plot(times, vacf_data,
           label='Velocity autocorrelation of the oxygen atoms')
pylab.xlabel('t (fs)')
pylab.ylabel('VACF(t)')
pylab.legend()

pylab.show()