Add support for 4- and 5-point water models with OpenMM

doc/source/changelog.rst

@@ -29,6 +29,8 @@ organisation on `GitHub <https://github.com/openbiosim/sire>`__.
 
 * Fix hang in ``sire.load`` function when shared GROMACS topology path is missing.
 
+* Add support for 4- and 5-point water models in the OpenMM conversion layer.
+
 `2025.4.0 <https://github.com/openbiosim/sire/compare/2025.3.0...2025.4.0>`__ - February 2026
 ---------------------------------------------------------------------------------------------
 

tests/convert/test_openmm_water.py

@@ -0,0 +1,197 @@
+import math
+
+import pytest
+import sire as sr
+
+_skip_no_openmm = pytest.mark.skipif(
+    "openmm" not in sr.convert.supported_formats(),
+    reason="openmm support is not available",
+)
+
+
+@pytest.fixture(scope="module")
+def tip3p_mols():
+    return sr.load_test_files("tip3p.s3")
+
+
+@pytest.fixture(scope="module")
+def tip4p_mols():
+    return sr.load_test_files("tip4p.s3")
+
+
+@pytest.fixture(scope="module")
+def tip5p_mols():
+    return sr.load_test_files("tip5p.s3")
+
+
+@pytest.fixture(scope="module")
+def opc_mols():
+    return sr.load_test_files("opc.s3")
+
+
+def _get_vsite_info(omm_context):
+    """Return (n_vsites, n_zero_mass, n_bad_constraints) for an OpenMM context."""
+    import openmm
+
+    sys = omm_context.getSystem()
+    natoms = sys.getNumParticles()
+
+    zero_mass = {
+        i
+        for i in range(natoms)
+        if sys.getParticleMass(i).value_in_unit(openmm.unit.dalton) == 0.0
+    }
+    n_vsites = sum(1 for i in zero_mass if sys.isVirtualSite(i))
+
+    bad_constraints = sum(
+        1
+        for k in range(sys.getNumConstraints())
+        if sys.getConstraintParameters(k)[0] in zero_mass
+        or sys.getConstraintParameters(k)[1] in zero_mass
+    )
+
+    return n_vsites, len(zero_mass), bad_constraints
+
+
+def _potential_energy(omm_context):
+    import openmm
+
+    return (
+        omm_context.getState(getEnergy=True)
+        .getPotentialEnergy()
+        .value_in_unit(openmm.unit.kilojoules_per_mole)
+    )
+
+
+@_skip_no_openmm
+def test_tip3p_no_virtual_sites(tip3p_mols, openmm_platform):
+    """TIP3P has no EP atoms — no virtual sites should be registered."""
+    omm_map = {
+        "constraint": "h-bonds",
+        "cutoff_type": "none",
+        "cutoff": "none",
+        "platform": openmm_platform or "CPU",
+    }
+
+    omm = sr.convert.to(tip3p_mols, "openmm", map=omm_map)
+    n_vsites, n_zero_mass, bad_constraints = _get_vsite_info(omm)
+
+    assert (
+        n_zero_mass == 0
+    ), f"TIP3P should have no zero-mass particles, got {n_zero_mass}"
+    assert n_vsites == 0, f"TIP3P should have no virtual sites, got {n_vsites}"
+    assert bad_constraints == 0
+
+    e = _potential_energy(omm)
+    assert not math.isnan(e), "Potential energy is NaN"
+
+
+@_skip_no_openmm
+def test_tip4p_virtual_sites(tip4p_mols, openmm_platform):
+    """TIP4P: one ThreeParticleAverageSite per water, no bad constraints, finite energy."""
+    import openmm
+
+    omm_map = {
+        "constraint": "h-bonds",
+        "cutoff_type": "none",
+        "cutoff": "none",
+        "platform": openmm_platform or "CPU",
+    }
+
+    omm = sr.convert.to(tip4p_mols, "openmm", map=omm_map)
+    n_vsites, n_zero_mass, bad_constraints = _get_vsite_info(omm)
+
+    n_waters = tip4p_mols.num_molecules()
+
+    assert (
+        n_zero_mass == n_waters
+    ), f"Expected {n_waters} zero-mass EP atoms, got {n_zero_mass}"
+    assert n_vsites == n_waters, f"Expected {n_waters} virtual sites, got {n_vsites}"
+    assert bad_constraints == 0, f"Constraints on virtual sites: {bad_constraints}"
+
+    # All virtual sites should be ThreeParticleAverageSite
+    sys = omm.getSystem()
+    natoms = sys.getNumParticles()
+    for i in range(natoms):
+        if sys.isVirtualSite(i):
+            vs = sys.getVirtualSite(i)
+            assert isinstance(
+                vs, openmm.ThreeParticleAverageSite
+            ), f"Particle {i}: expected ThreeParticleAverageSite, got {type(vs).__name__}"
+
+    e = _potential_energy(omm)
+    assert not math.isnan(e), "Potential energy is NaN"
+
+
+@_skip_no_openmm
+def test_opc_virtual_sites(opc_mols, openmm_platform):
+    """OPC: one ThreeParticleAverageSite per water, no bad constraints, finite energy."""
+    import openmm
+
+    omm_map = {
+        "constraint": "h-bonds",
+        "cutoff_type": "none",
+        "cutoff": "none",
+        "platform": openmm_platform or "CPU",
+    }
+
+    omm = sr.convert.to(opc_mols, "openmm", map=omm_map)
+    n_vsites, n_zero_mass, bad_constraints = _get_vsite_info(omm)
+
+    n_waters = opc_mols.num_molecules()
+
+    assert (
+        n_zero_mass == n_waters
+    ), f"Expected {n_waters} zero-mass EP atoms, got {n_zero_mass}"
+    assert n_vsites == n_waters, f"Expected {n_waters} virtual sites, got {n_vsites}"
+    assert bad_constraints == 0, f"Constraints on virtual sites: {bad_constraints}"
+
+    sys = omm.getSystem()
+    natoms = sys.getNumParticles()
+    for i in range(natoms):
+        if sys.isVirtualSite(i):
+            vs = sys.getVirtualSite(i)
+            assert isinstance(
+                vs, openmm.ThreeParticleAverageSite
+            ), f"Particle {i}: expected ThreeParticleAverageSite, got {type(vs).__name__}"
+
+    e = _potential_energy(omm)
+    assert not math.isnan(e), "Potential energy is NaN"
+
+
+@_skip_no_openmm
+def test_tip5p_virtual_sites(tip5p_mols, openmm_platform):
+    """TIP5P: two OutOfPlaneSite virtual sites per water, no bad constraints, finite energy."""
+    import openmm
+
+    omm_map = {
+        "constraint": "h-bonds",
+        "cutoff_type": "none",
+        "cutoff": "none",
+        "platform": openmm_platform or "CPU",
+    }
+
+    omm = sr.convert.to(tip5p_mols, "openmm", map=omm_map)
+    n_vsites, n_zero_mass, bad_constraints = _get_vsite_info(omm)
+
+    n_waters = tip5p_mols.num_molecules()
+
+    assert (
+        n_zero_mass == 2 * n_waters
+    ), f"Expected {2 * n_waters} zero-mass EP atoms, got {n_zero_mass}"
+    assert (
+        n_vsites == 2 * n_waters
+    ), f"Expected {2 * n_waters} virtual sites, got {n_vsites}"
+    assert bad_constraints == 0, f"Constraints on virtual sites: {bad_constraints}"
+
+    sys = omm.getSystem()
+    natoms = sys.getNumParticles()
+    for i in range(natoms):
+        if sys.isVirtualSite(i):
+            vs = sys.getVirtualSite(i)
+            assert isinstance(
+                vs, openmm.OutOfPlaneSite
+            ), f"Particle {i}: expected OutOfPlaneSite, got {type(vs).__name__}"
+
+    e = _potential_energy(omm)
+    assert not math.isnan(e), "Potential energy is NaN"