add diffraction_pattern method and unit test

cmeutils/structure.py

@@ -476,7 +476,7 @@ def structure_factor(
     method="direct",
     ref_length=None,
 ):
-    """
+    """Uses freud's diffraction module to find 1D structure factors.
 
     Parameters
     ----------
@@ -508,6 +508,9 @@ def structure_factor(
     freud.diffraction.StaticStructureFactorDebye
     """
 
+    if not ref_length:
+        ref_length = 1
+
     if method.lower() == "direct":
         sf = freud.diffraction.StaticStructureFactorDirect(
             bins=bins, k_max=k_max, k_min=k_min
@@ -520,15 +523,59 @@ def structure_factor(
         raise ValueError(
             f"Optional methods are `debye` or `direct`, you chose {method}"
         )
-    if not ref_length:
-        ref_length = 1
     with gsd.hoomd.open(gsdfile, mode="r") as trajectory:
         for frame in trajectory[start:stop]:
             system = frame_to_freud_system(frame=frame, ref_length=ref_length)
             sf.compute(system=system, reset=False)
     return sf
 
 
+def diffraction_pattern(
+    gsdfile,
+    views,
+    start=0,
+    stop=-1,
+    ref_length=None,
+    grid_size=1024,
+    output_size=None,
+):
+    """Uses freud's diffraction module to find 2D diffraction patterns.
+
+    Parameters
+    ----------
+    gsdfile : str, required
+        File path to the GSD trajectory.
+    views : list, required
+        List of orientations (quarternions) to average over.
+        See cmeutils.structure.get_quarternions
+    start : int, default 0
+        Starting frame index for accumulating the Sq. Negative numbers index
+        from the end.
+    stop : int, optional default None
+        Final frame index for accumulating the Sq. If None, the last frame
+        will be used.
+    ref_length : float, optional, default None
+        Set a reference length to convert from reduced units to real units.
+        If None, uses 1 by default.
+
+    Returns
+    -------
+    freud.diffraction.DiffractionPattern
+    """
+
+    if not ref_length:
+        ref_length = 1
+    dp = freud.diffraction.DiffractionPattern(
+        grid_size=grid_size, output_size=output_size
+    )
+    with gsd.hoomd.open(gsdfile) as trajectory:
+        for frame in trajectory[start:stop]:
+            system = frame_to_freud_system(frame=frame, ref_length=ref_length)
+            for view in views:
+                dp.compute(system=system, view_orientation=view, reset=False)
+    return dp
+
+
 def get_centers(gsdfile, new_gsdfile):
     """Create a gsd file containing the molecule centers from an existing gsd
      file.

cmeutils/tests/test_structure.py

@@ -8,6 +8,7 @@
     all_atom_rdf,
     angle_distribution,
     bond_distribution,
+    diffraction_pattern,
     dihedral_distribution,
     get_centers,
     get_quaternions,
@@ -187,6 +188,11 @@ def test_angle_range_outside(self, p3ht_gsd):
                 theta_max=180,
             )
 
+    def test_diffraction_pattern(self, gsdfile_bond):
+        views = get_quaternions(n_views=5)
+        dp = diffraction_pattern(gsdfile_bond, views=views)
+        assert isinstance(dp, freud.diffraction.DiffractionPattern)
+
     def test_structure_factor_direct(self, gsdfile_bond):
         sf = structure_factor(gsdfile_bond, k_min=0.2, k_max=5)
         assert isinstance(sf, freud.diffraction.StaticStructureFactorDirect)