fix(ase): symmetrize reshaped virial stress

deepmd/calculator.py

@@ -152,13 +152,17 @@ def calculate(
         # see https://gitlab.com/ase/ase/-/merge_requests/2485
         self.results["free_energy"] = e[0][0]
         self.results["forces"] = f[0]
-        self.results["virial"] = v[0].reshape(3, 3)
+        virial = v[0].reshape(3, 3)
+        self.results["virial"] = virial
 
         # convert virial into stress for lattice relaxation
         if cell is not None:
             # the usual convention (tensile stress is positive)
             # stress = -virial / volume
-            stress = -0.5 * (v[0].copy() + v[0].copy().T) / atoms.get_volume()
+            # ASE represents Cauchy stress as a symmetric tensor.  Reshape the
+            # flat model output before transposing; ``.T`` on the original
+            # one-dimensional virial array would otherwise be a no-op.
+            stress = -0.5 * (virial + virial.T) / atoms.get_volume()
             # Voigt notation
             self.results["stress"] = stress.flat[[0, 4, 8, 5, 2, 1]]
         elif "stress" in properties:

source/tests/test_calculator.py

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: LGPL-3.0-or-later
+"""Backend-independent regression tests for the ASE calculator adapter."""
+
+from unittest.mock import (
+    Mock,
+    patch,
+)
+
+import numpy as np
+from ase import (
+    Atoms,
+)
+
+from deepmd.calculator import (
+    DP,
+)
+
+
+def test_stress_symmetrizes_flat_virial() -> None:
+    """Convert the model virial to ASE's symmetric Voigt stress convention.
+
+    Real DeePMD models normally produce an already-symmetric total virial, so
+    the existing integration tests could not detect that transposing its flat
+    nine-component representation was a no-op.  An intentionally asymmetric
+    mock output makes each off-diagonal average independently observable.
+    """
+    model = Mock()
+    model.get_type_map.return_value = ["H"]
+    model.get_ntypes.return_value = 1
+    virial = np.arange(1.0, 10.0).reshape(1, 9)
+    model.eval.return_value = (
+        np.array([[0.0]]),
+        np.zeros((1, 1, 3)),
+        virial,
+    )
+
+    with patch("deepmd.calculator.DeepPot", return_value=model):
+        calculator = DP("unused-model")
+
+    atoms = Atoms(
+        "H",
+        positions=[[0.0, 0.0, 0.0]],
+        cell=np.eye(3) * 2.0,
+        pbc=True,
+        calculator=calculator,
+    )
+
+    np.testing.assert_allclose(
+        atoms.get_stress(voigt=True),
+        -np.array([1.0, 5.0, 9.0, 7.0, 5.0, 3.0]) / atoms.get_volume(),
+    )
+    # Symmetrizing stress must not discard diagnostic information from the
+    # model: callers requesting the virial still receive the original tensor.
+    np.testing.assert_array_equal(calculator.results["virial"], virial.reshape(3, 3))