Velocity autocorrelation function for molecular dynamics trajectory

I have a 1 ns long molecular dynamics trajectory of 884 molecules of water, and I am trying to compute the velocity autocorrelation function using MDAnalysis to analyze the GROMACS output files.

I have written:

def vacf(ncorr):    acf, norm = np.zeros(ncorr), np.zeros(ncorr) # initialize autocorrelation function and normalization arrays    n = 0 # first frame    for ts in u.trajectory: # loop through the trajectory        vel = u.trajectory[n].velocities.flatten() # calculate velocities at t0        maxn = min(u.trajectory.n_frames, n + ncorr) - n        for i in range(maxn):            veln = u.trajectory[n + i].velocities.flatten() # compute velocities at time t + t0            acf[i] = acf[i] + np.dot(vel, veln.T) / (len(u.atoms))            norm[i] = norm[i] + 1        n += 1    acf = acf / norm    return acf

The velocity autocorrelation function results are usually something that start at 1 and slowly decay to 0, but this is not the case for the simulation I'm analyzing. The output I'm getting is:

Do you have any ideas on what I'm doing wrong?Thank you very much in advance, have a great day!