fix(vasp): interpret negative POSCAR scale as volume

dpdata/formats/vasp/poscar.py

@@ -18,6 +18,18 @@ def move_flag_mapper(flag):
     system["atom_names"] = [str(ii) for ii in lines[5].split()]
     system["atom_numbs"] = [int(ii) for ii in lines[6].split()]
     scale = float(lines[1])
+    # VASP interprets a negative scale as a target cell volume (in A^3), not
+    # as a negative vector multiplier.  Convert it to a positive linear scale
+    # after reading the unscaled lattice vectors.
+    if scale < 0:
+        target_volume = abs(scale)
+        raw_cell = np.array(
+            [[float(jj) for jj in lines[ii].split()] for ii in range(2, 5)]
+        )
+        raw_volume = abs(np.linalg.det(raw_cell))
+        if raw_volume == 0:
+            raise ValueError("negative POSCAR scale requires a non-zero cell volume")
+        scale = (target_volume / raw_volume) ** (1.0 / 3.0)
     cell = []
     move_flags = []
     for ii in range(2, 5):

tests/test_vasp_poscar_to_system.py

@@ -8,6 +8,8 @@
 from context import dpdata
 from poscars.poscar_ref_oh import TestPOSCARoh
 
+from dpdata.formats.vasp.poscar import to_system_data
+
 
 class TestPOSCARCart(unittest.TestCase, TestPOSCARoh):
     def setUp(self):
@@ -89,5 +91,54 @@ def setUp(self):
         self.v_places = 6
 
 
+class TestVaspNegativeScale(unittest.TestCase):
+    def test_negative_scale_is_target_volume(self):
+        lines = """test
+-8
+1 0 0
+0 1 0
+0 0 1
+H
+1
+Direct
+0.5 0.0 0.0
+""".splitlines()
+        data = to_system_data(lines)
+        np.testing.assert_allclose(np.linalg.det(data["cells"][0]), 8.0)
+        np.testing.assert_allclose(data["coords"][0, 0], [1.0, 0.0, 0.0])
+
+    def test_negative_scale_rescales_cartesian_coordinates(self):
+        lines = """test
+-8
+1 0 0
+0 1 0
+0 0 1
+H
+1
+Cartesian
+0.25 0.5 0.75
+""".splitlines()
+
+        data = to_system_data(lines)
+
+        np.testing.assert_allclose(np.linalg.det(data["cells"][0]), 8.0)
+        np.testing.assert_allclose(data["coords"][0, 0], [0.5, 1.0, 1.5])
+
+    def test_negative_scale_rejects_zero_volume_cell(self):
+        lines = """test
+-8
+1 0 0
+0 0 0
+0 0 1
+H
+1
+Direct
+0.5 0.0 0.0
+""".splitlines()
+
+        with self.assertRaisesRegex(ValueError, "non-zero cell volume"):
+            to_system_data(lines)
+
+
 if __name__ == "__main__":
     unittest.main()