Relax criteria for structure to be the same. Now just raises warning.

Further added tests to ensure correct behavior. Fixes #1615.

pymatgen/io/vasp/outputs.py

@@ -2922,6 +2922,17 @@ def __add__(self, other):
     def __sub__(self, other):
         return self.linear_add(other, -1.0)
 
+    def copy(self):
+        """
+        :return: Copy of Volumetric object
+        """
+        return VolumetricData(
+            self.structure,
+            {k: v.copy() for k, v in self.data.items()},
+            distance_matrix=self._distance_matrix,
+            data_aug=self.data_aug
+        )
+
     def linear_add(self, other, scale_factor=1.0):
         """
         Method to do a linear sum of volumetric objects. Used by + and -
@@ -2936,9 +2947,12 @@ def linear_add(self, other, scale_factor=1.0):
             VolumetricData corresponding to self + scale_factor * other.
         """
         if self.structure != other.structure:
-            raise ValueError("Adding or subtraction operations can only be "
-                             "performed for volumetric data with the exact "
-                             "same structure.")
+            warnings.warn("Structures are different. Make sure you know what "
+                          "you are doing...")
+        if self.data.keys() != other.data.keys():
+            raise ValueError("Data have different keys! Maybe one is spin-"
+                             "polarized and the other is not?")
+
         # To add checks
         data = {}
         for k in self.data.keys():

pymatgen/io/vasp/tests/test_outputs.py

@@ -1135,6 +1135,22 @@ def test_spin_data(self):
         for k, v in d.items():
             self.assertEqual(v.shape, (48, 48, 48))
 
+    def test_add(self):
+        chgcar_sum = self.chgcar_spin + self.chgcar_spin
+        self.assertArrayAlmostEqual(chgcar_sum.data['total'], self.chgcar_spin.data['total'] * 2)
+        chgcar_copy = self.chgcar_spin.copy()
+        chgcar_copy.structure = self.get_structure("Li2O")
+        with warnings.catch_warnings(record=True) as w:
+            # Cause all warnings to always be triggered.
+            warnings.simplefilter("always")
+            # Trigger a warning.
+            chgcar_sum = chgcar_copy + self.chgcar_spin
+            # Verify some things
+            assert len(w) == 1
+            assert "Structures are different. Make sure you know what you are doing..." in str(w[-1].message)
+        self.assertRaises(ValueError, self.chgcar_spin.__add__, self.chgcar_fe3o4)
+        self.assertRaises(ValueError, self.chgcar_spin.__add__, self.chgcar_no_spin)
+
     def test_as_dict_and_from_dict(self):
         d = self.chgcar_NiO_SOC.as_dict()
         chgcar_from_dict = Chgcar.from_dict(d)