Fix force loss when v-sites are present

smee/geometry.py

@@ -199,27 +199,27 @@ def _build_v_site_coord_frames(
         origin = weighted_coords[:, 0, :]
 
         xy_plane = weighted_coords[:, 1:, :]
-        xy_plane /= torch.norm(xy_plane, dim=-1).unsqueeze(-1)
+        xy_plane_hat = xy_plane / torch.norm(xy_plane, dim=-1).unsqueeze(-1)
 
-        x_hat = xy_plane[:, 0, :]
+        x_hat = xy_plane_hat[:, 0, :]
 
-        z_hat = torch.cross(x_hat, xy_plane[:, 1, :])
-        z_hat_norm = torch.norm(z_hat, dim=-1).unsqueeze(-1)
-        z_hat_norm = torch.where(
-            torch.isclose(z_hat_norm, smee.utils.tensor_like(0.0, other=z_hat_norm)),
-            smee.utils.tensor_like(1.0, other=z_hat_norm),
-            z_hat_norm,
+        z = torch.cross(x_hat, xy_plane_hat[:, 1, :])
+        z_norm = torch.norm(z, dim=-1).unsqueeze(-1)
+        z_norm_clamped = torch.where(
+            torch.isclose(z_norm, smee.utils.tensor_like(0.0, other=z_norm)),
+            smee.utils.tensor_like(1.0, other=z_norm),
+            z_norm,
         )
-        z_hat /= z_hat_norm
-
-        y_hat = torch.cross(z_hat, x_hat)
-        y_hat_norm = torch.norm(y_hat, dim=-1).unsqueeze(-1)
-        y_hat_norm = torch.where(
-            torch.isclose(y_hat_norm, smee.utils.tensor_like(0.0, other=y_hat_norm)),
-            smee.utils.tensor_like(1.0, other=y_hat_norm),
-            y_hat_norm,
+        z_hat = z / z_norm_clamped
+
+        y = torch.cross(z_hat, x_hat)
+        y_norm = torch.norm(y, dim=-1).unsqueeze(-1)
+        y_norm_clamped = torch.where(
+            torch.isclose(y_norm, smee.utils.tensor_like(0.0, other=y_norm)),
+            smee.utils.tensor_like(1.0, other=y_norm),
+            y_norm,
         )
-        y_hat /= y_hat_norm
+        y_hat = y / y_norm_clamped
 
         stacked_frames[0].append(origin)
         stacked_frames[1].append(x_hat)

smee/tests/test_geometry.py

@@ -352,3 +352,29 @@ def test_add_v_site_coords(conformer, v_site_coords, cat_dim, mocker):
 
     assert coordinates.shape == expected_coords.shape
     assert torch.allclose(coordinates, expected_coords)
+
+
+def test_add_v_site_coords_grad(v_site_force_field):
+    """Test that the gradients of functions of v-site coordinates can be computed,
+    and also gradients of the gradients (e.g. loss of forces involving v-sites). This
+    was found to be a bug upstream, yeilding a tensor modified in place error."""
+    molecule = openff.toolkit.Molecule.from_mapped_smiles("[H:2][O:1][H:3]")
+    molecule.generate_conformers(n_conformers=1)
+
+    conformer = torch.tensor(molecule.conformers[0].m_as(unit.angstrom))
+    conformer.requires_grad_(True)
+
+    interchange = openff.interchange.Interchange.from_smirnoff(
+        v_site_force_field, molecule.to_topology()
+    )
+    force_field, [topology] = smee.converters.convert_interchange(interchange)
+
+    assert topology.v_sites is not None
+    assert len(topology.v_sites.keys) > 0
+
+    v_site_coords = add_v_site_coords(topology.v_sites, conformer, force_field)
+
+    grad = torch.autograd.grad(v_site_coords.sum(), conformer, create_graph=True)[0]
+
+    loss = grad.sum()
+    loss.backward()