VMD Numpy

The vmdnumpy module exposes molecular coordinates in-memory as a numpy array. This allows extremely efficient analyses to be conducted.

Take care– the coordinates given are the same as those in memory. Any inadveretent modification will require you to reload your trajectory to restore the original values! However, this can be very useful in implementing trajectory smoothing functions, etc.

atomselect	Return an array of ints representing flags for on/off atoms in an atom selection
positions	Return a zero-copy reference to atomic coordinates
timestep	Return a zero-copy reference to atomic coordinates
velocities	Return a zero-copy reference to atomic velocites, or None if no velocity information is present

For example, the following code will apply a Savitsky-Golay smoothing function to the entire trajectory:

import numpy as np
from scipy.signal import savgol_filter
from vmd import molecule, vmdnumpy

def smooth_trajectory(molid, window=5, polyorder=3):
    smoother = np.empty((molecule.numframes(molid), molecule.numatoms(molid)*3))

    # Copy timestep data to a 1D array to run filter on
    for t in range(molecule.numframes(molid)):
        smoother[t] = vmdnumpy.timestep(molid, t).flatten()
    smoothed = savgol_filter(smoother, window, polyorder,
                             axis=0, mode="nearest")

    # Copy filtered data back into coordinates
    for t in range(molecule.numframes(molid)):
        conv = smoothed[t].reshape((molecule.numatoms(molid), 3))
        np.copyto(vmdnumpy.timestep(molid, t), conv)