Raise an error when faced with ambiguous CASTEP .cell

CASTEP and Pymatgen orientation are likely to be different from the
same lattice vectors. To avoid building incorrect structures, refuse
to work with lattice_abc and positions_abs simultaneously.

sumo/io/castep.py

@@ -100,9 +100,11 @@ def structure(self):
             return Structure(lattice, elements, coords, coords_are_cartesian=False)
         elif "positions_abs" in self.blocks:
             if "lattice_abc" in self.blocks:
-                logger.warning(
-                    "Positions are given in absolute coordinates and lattice "
-                    "in angles/lengths format. Structure may not be consistent."
+                raise ValueError(
+                    "Positions are given in Cartesian coordinates and lattice "
+                    "in angles/lengths format. This structure cannot be "
+                    "interpreted reliably. Consider using c2x to convert to "
+                    "fractional coordinates and/or 3x3 lattice matrix."
                 )
 
             positions_abs = self.blocks["positions_abs"].values

tests/data/NiO/NiO_abc.cell

@@ -3,12 +3,12 @@
 106.80215981  73.19772637 119.99807072
 %ENDBLOCK LATTICE_ABC
 
-%BLOCK POSITIONS_ABS
+%BLOCK POSITIONS_FRAC
 Ni  0.000000  0.000000  0.000000 SPIN=1
-Ni  1.491603  0.861143  2.432002 SPIN=-1
-O  0.000069  1.722313  1.215967  SPIN=0
-O  2.983138 -0.000026  3.648036  SPIN=0
-%ENDBLOCK POSITIONS_ABS
+Ni  0.500000  0.500000  0.500000 SPIN=-1
+O  0.250001  0.749999  0.249993  SPIN=0
+O  0.749999  0.250001  0.750007  SPIN=0
+%ENDBLOCK POSITIONS_FRAC
 
 %BLOCK SPECIES_POT
 Ni C19

tests/data/NiO/NiO_abc_units.cell

@@ -4,13 +4,12 @@ Ang
 106.80215981  73.19772637 119.99807072
 %ENDBLOCK LATTICE_ABC
 
-%BLOCK POSITIONS_ABS
-Ang
+%BLOCK POSITIONS_FRAC
 Ni  0.000000  0.000000  0.000000 SPIN=1
-Ni  1.491603  0.861143  2.432002 SPIN=-1
-O  0.000069  1.722313  1.215967  SPIN=0
-O  2.983138 -0.000026  3.648036  SPIN=0
-%ENDBLOCK POSITIONS_ABS
+Ni  0.500000  0.500000  0.500000 SPIN=-1
+O  0.250001  0.749999  0.249993  SPIN=0
+O  0.749999  0.250001  0.750007  SPIN=0
+%ENDBLOCK POSITIONS_FRAC
 
 %BLOCK SPECIES_POT
 Ni C19
@@ -19,4 +18,3 @@ O  C19
 
 KPOINTS_MP_GRID 7 7 7
 SPECTRAL_KPOINTS_MP_GRID 2 2 2
-

tests/tests_io/test_castep.py

@@ -38,12 +38,19 @@ def setUp(self):
         self.zns_singlepoint_cell = os.path.join(
             ilr_files("tests"), "data", "ZnS", "zns-sp.cell"
         )
+        self.nio_cart_cell = os.path.join(
+            ilr_files("tests"), "data", "NiO", "NiO.cell"
+        )
         self.nio_abc_cell = os.path.join(
             ilr_files("tests"), "data", "NiO", "NiO_abc.cell"
         )
         self.nio_abc_units_cell = os.path.join(
             ilr_files("tests"), "data", "NiO", "NiO_abc_units.cell"
         )
+        self.nio_abc_cart_cell = os.path.join(
+            ilr_files("tests"), "data", "NiO", "NiO_abc_cart.cell"
+        )
+
 
     def test_castep_cell_null_init(self):
         null_cell = CastepCell()
@@ -82,20 +89,33 @@ def test_castep_cell_from_singlepoint_file(self):
     def test_castep_cell_abc(self):
         """Test .cell file using lattice_abc to define unit cell"""
         for filename in self.nio_abc_cell, self.nio_abc_units_cell:
-            with self.assertLogs(logger) as log:
-                cc = CastepCell.from_file(filename)
-                structure = cc.structure
-                self.assertIn("Structure may not be consistent.", log.output[-1])
+            cc = CastepCell.from_file(filename)
+            structure = cc.structure
 
             assert_array_almost_equal(
-                structure.lattice.matrix,
-                [
-                    [2.855724, 0.0, 0.862317],
-                    [-1.297582, 2.54391, -0.862313],
-                    [0.0, 0.0, 5.159941],
-                ],
+                structure.lattice.abc,
+                [2.983077, 2.98308198, 5.15994087]
+            )
+            assert_array_almost_equal(
+                structure.lattice.angles,
+                [106.80215981, 73.19772637, 119.99807072]
             )
 
+    def test_castep_cell_consistent_frac_coords(self):
+        """Test fractional positions are consistent between .cell formats"""
+        cartesian_cell = CastepCell.from_file(self.nio_cart_cell)
+        frac_abc_cell = CastepCell.from_file(self.nio_abc_cell)
+
+        assert_array_almost_equal(
+            cartesian_cell.structure.frac_coords,
+            frac_abc_cell.structure.frac_coords
+        )
+
+    def test_castep_abc_cart_raises(self):
+        """Test that error is raised for incompatible vectors and positions"""
+        with self.assertRaisesRegex(ValueError, "Cartesian"):
+            CastepCell.from_file(self.nio_abc_cart_cell).structure
+
     def test_castep_cell_from_structure(self):
         cell = CastepCell.from_structure(self.si_structure)
         self.assertEqual(