[dG] Fix extracting solvent when v-sites present on solute (#126)

smee/mm/_fe.py

@@ -47,19 +47,16 @@ def _extract_pure_solvent(
     device = force_field.potentials[0].parameters.device
     dtype = force_field.potentials[0].parameters.dtype
 
-    system, beta, pressure, u_kn, n_k, xyz, box = _load_samples(
+    system, _, _, _, _, xyz, box = _load_samples(
         solute, solvent, output_dir, device, dtype, coord_state_idx=-1
     )
 
     if len(system.topologies) != 2 or system.n_copies[0] != 1:
         raise NotImplementedError("only single solute systems are supported.")
 
-    n_solute_atoms = system.topologies[0].n_atoms
-    xyz = xyz[:, n_solute_atoms:, :]
+    xyz = xyz[:, solute.n_particles :, :]
 
-    system = smee.TensorSystem(
-        [system.topologies[1]], [system.n_copies[1]], is_periodic=True
-    )
+    system = smee.TensorSystem([solvent], [system.n_copies[1]], is_periodic=True)
     energy = _compute_energy(system, force_field, xyz, box)
 
     return xyz, box, energy

smee/tests/mm/test_fe.py

@@ -2,16 +2,35 @@
 
 import openff.interchange
 import openff.toolkit
+import openff.units
+import openmm.unit
 import pytest
 import torch
 
 import smee.converters
 import smee.mm
+import smee.mm._fe
 
 
 def load_systems(solute: str, solvent: str):
     ff_off = openff.toolkit.ForceField("openff-2.0.0.offxml")
 
+    v_site_handler = ff_off.get_parameter_handler("VirtualSites")
+    v_site_handler.add_parameter(
+        {
+            "type": "DivalentLonePair",
+            "match": "once",
+            "smirks": "[*:2][#7:1][*:3]",
+            "distance": 0.4 * openff.units.unit.angstrom,
+            "epsilon": 0.0 * openff.units.unit.kilojoule_per_mole,
+            "sigma": 0.1 * openff.units.unit.nanometer,
+            "outOfPlaneAngle": 0.0 * openff.units.unit.degree,
+            "charge_increment1": 0.0 * openff.units.unit.elementary_charge,
+            "charge_increment2": 0.0 * openff.units.unit.elementary_charge,
+            "charge_increment3": 0.0 * openff.units.unit.elementary_charge,
+        }
+    )
+
     solute_inter = openff.interchange.Interchange.from_smirnoff(
         ff_off,
         openff.toolkit.Molecule.from_smiles(solute).to_topology(),
@@ -29,6 +48,28 @@ def load_systems(solute: str, solvent: str):
     return top_solute, top_solvent, ff
 
 
+def test_extract_pure_solvent(tmp_cwd, mocker):
+    top_solute, top_solvent, ff = load_systems("c1ccncc1", "O")
+
+    system = smee.TensorSystem([top_solute, top_solvent], [1, 10], True)
+    xyz, box = smee.mm.generate_system_coords(system, ff)
+
+    xyz = torch.tensor(xyz.value_in_unit(openmm.unit.angstrom)).unsqueeze(0)
+    box = torch.tensor(box.value_in_unit(openmm.unit.angstrom)).unsqueeze(0) * 10.0
+
+    mocker.patch(
+        "smee.mm._fe._load_samples",
+        return_value=(system, None, None, None, None, xyz, box),
+    )
+
+    xyz_solv, _, _ = smee.mm._fe._extract_pure_solvent(
+        top_solute, top_solvent, ff, tmp_cwd
+    )
+
+    assert xyz_solv.shape == (1, 30, 3)
+    assert torch.allclose(xyz_solv, xyz[:, 12:, :])
+
+
 @pytest.mark.fe
 def test_fe_ops(tmp_cwd):
     # taken from a run on commit ec3d272b466f761ed838e16a5ba7b97ceadc463b