enforced double precision calculations in *Group.center()

package/CHANGELOG

@@ -32,6 +32,9 @@ Enhancements
   * Added *Group.copy() methods returning an identical copy of the respective
     group (PR #1922)
   * Use a faster function to deduplicate indices (PR #1951)
+  * Calculations in *Group.center() are performed in double precision (#PR1936)
+  * Functions in lib.distances accept coordinate arrays of arbitrary dtype
+    (PR #1936)
 
 Fixes
   * rewind in the SingleFrameReader now reads the frame from the file (Issue #1929)

package/MDAnalysis/core/groups.py

@@ -711,11 +711,19 @@ def center(self, weights, pbc=None, compound='group'):
 
         atoms = self.atoms
 
+        # enforce calculations in double precision:
+        dtype = np.float64
+
         if compound.lower() == 'group':
             if pbc:
                 coords = atoms.pack_into_box(inplace=False)
             else:
                 coords = atoms.positions
+            # promote coords or weights to dtype if required:
+            if weights is None:
+                coords = coords.astype(dtype, copy=False)
+            else:
+                weights = weights.astype(dtype, copy=False)
             return np.average(coords, weights=weights, axis=0)
         elif compound.lower() == 'residues':
             compound_indices = atoms.resindices
@@ -728,14 +736,18 @@ def center(self, weights, pbc=None, compound='group'):
                              " one of 'group', 'residues', or 'segments'."
                              "".format(compound))
 
-        # Sort positions and masses by compound index:
+        # Sort positions and weights by compound index and promote to dtype if
+        # required:
         sort_indices = np.argsort(compound_indices)
         compound_indices = compound_indices[sort_indices]
         coords = atoms.positions[sort_indices]
-        if weights is not None:
+        if weights is None:
+            coords = coords.astype(dtype, copy=False)
+        else:
+            weights = weights.astype(dtype, copy=False)
             weights = weights[sort_indices]
         # Allocate output array:
-        centers = np.zeros((n_compounds, 3), dtype=coords.dtype)
+        centers = np.zeros((n_compounds, 3), dtype=dtype)
         # Get sizes of compounds:
         unique_compound_indices, compound_sizes = np.unique(compound_indices,
                                                             return_counts=True)