diff --git a/MDANSE/Src/MDANSE/Mathematics/Signal.py b/MDANSE/Src/MDANSE/Mathematics/Signal.py
index 667ff0431..bab2a1e4d 100644
--- a/MDANSE/Src/MDANSE/Mathematics/Signal.py
+++ b/MDANSE/Src/MDANSE/Mathematics/Signal.py
@@ -20,6 +20,7 @@
 from functools import partial
 
 import scipy.signal
+from MDANSE.Mathematics.Arithmetic import weight
 from scipy import signal, fftpack
 
 from MDANSE.Core.Error import Error
@@ -816,6 +817,84 @@ def __init__(self, **kwargs):
 
 filter_map = {filter_class.__name__: filter_class for filter_class in FILTERS}
 
+def power_spectrum(
+        trajectory,
+        frames,
+        projection,
+        atom_selection,
+        weights,
+        instrument_resolution,
+        n_steps
+):
+    """
+
+    """
+    trajectory = trajectory["instance"]
+    sorted_atoms = trajectory.chemical_system.atom_list
+
+    output = dict()
+    output["romega"] = instrument_resolution["romega"]
+
+
+
+    for element in atom_selection["unique_names"]:
+        output["pacf_%s" % element] = np.zeros(np.array(range(frames["n_frames"])).shape)
+        output["pps_%s" % element] = np.zeros(output["romega"].shape)
+
+    output["pacf_total"] = np.zeros(np.array(range(frames["n_frames"])).shape)
+    output["pps_total"] = np.zeros(output["romega"].shape)
+
+    for index in range(n_steps):
+        indexes = atom_selection["indexes"][index]
+        atoms = [sorted_atoms[idx] for idx in indexes]
+        series = trajectory.read_com_trajectory(
+            atoms,
+            first=frames["first"],
+            last=frames["last"] + 1,
+            step=frames["step"],
+        )
+
+        series = series - np.average(series, axis=0)
+        series = projection["projector"](series)
+
+        n_configs = frames["n_configs"]
+        atomicPACF = signal.correlate(series, series[:n_configs], mode="valid") / (
+            3 * n_configs
+        )
+
+        output["pacf_%s" % atom_selection["names"][index]] += np.array([x[0] for x in atomicPACF])
+
+    nAtomsPerElement = atom_selection.get_natoms()
+    for element, number in nAtomsPerElement.items():
+        output["pacf_%s" % element][:] /= number
+        output["pps_%s" % element][:] = get_spectrum(
+            output["pacf_%s" % element],
+            instrument_resolution["time_window"],
+            instrument_resolution["time_step"],
+            fft="rfft"
+        )
+
+    weights = weights.get_weights()
+
+    output["pacf_total"][:] = weight(
+        weights,
+        output,
+        nAtomsPerElement,
+        1,
+        "pacf_%s",
+        update_partials=True,
+    )
+    output["pps_total"][:] = weight(
+        weights,
+        output,
+        nAtomsPerElement,
+        1,
+        "pps_%s",
+        update_partials=True,
+    )
+
+    # Adjust to atom selection
+    return (output["romega"], output["pps_total"])
 
 if __name__=="__main__":
     filter_class = filter_map["ChebyshevTypeI"]