Fix virial sign in NPT ensemble, wrap coordinates to unit cell, and support charge/spin configuration

src/fairchem/lammps/lammps_fc.py

@@ -8,6 +8,7 @@
 import numpy as np
 import torch
 from ase.data import atomic_masses, chemical_symbols
+from ase.geometry import wrap_positions
 
 from fairchem.core.datasets.atomic_data import AtomicData
 from lammps import lammps
@@ -32,16 +33,23 @@ def check_input_script(input_script: str):
 
 def check_atom_id_match_masses(types_arr, masses):
     for atom_id in types_arr:
-        assert np.allclose(
-            masses[atom_id], atomic_masses[atom_id], atol=1e-1
-        ), f"Atom {chemical_symbols[atom_id]} (type {atom_id}) has mass {masses[atom_id]} but is expected to have mass {atomic_masses[atom_id]}."
+        assert np.allclose(masses[atom_id], atomic_masses[atom_id], atol=1e-1), (
+            f"Atom {chemical_symbols[atom_id]} (type {atom_id}) has mass {masses[atom_id]} but is expected to have mass {atomic_masses[atom_id]}."
+        )
 
 
 def atomic_data_from_lammps_data(
-    x, atomic_numbers, nlocal, cell, periodicity, task_name
+    x: np.ndarray | torch.Tensor,
+    atomic_numbers,
+    nlocal,
+    cell,
+    periodicity,
+    task_name,
+    charge: int = 0,
+    spin: int = 0,
 ):
     # TODO: do we need to take of care of wrapping atoms that are outside the cell?
-    pos = torch.tensor(x, dtype=torch.float32)
+    pos = torch.as_tensor(x, dtype=torch.float32)
     pbc = torch.tensor(periodicity, dtype=torch.bool).unsqueeze(0)
     edge_index = torch.empty((2, 0), dtype=torch.long)
     cell_offsets = torch.empty((0, 3), dtype=torch.float32)
@@ -58,8 +66,8 @@ def atomic_data_from_lammps_data(
         edge_index=edge_index,
         cell_offsets=cell_offsets,
         nedges=nedges,
-        charge=torch.LongTensor([0]),
-        spin=torch.LongTensor([0]),
+        charge=torch.LongTensor([charge]),
+        spin=torch.LongTensor([spin]),
         fixed=fixed,
         tags=tags,
         batch=batch,
@@ -116,7 +124,7 @@ def lookup_atomic_number_by_mass(mass_arr: np.ndarray | float) -> np.ndarray | i
     return atomic_numbers
 
 
-def separate_run_commands(input_script: str) -> str:
+def separate_run_commands(input_script: str) -> tuple[list[str], list[str]]:
     lines = input_script.splitlines()
     run_cmds = []
     script = []
@@ -145,52 +153,74 @@ def cell_from_lammps_box(boxlo, boxhi, xy, yz, xz):
     return unit_cell_matrix.unsqueeze(0)
 
 
-def fix_external_call_back(lmp, ntimestep, nlocal, tag, x, f):
-    # force copy here, otherwise we can accident modify the original array in lammps
-    # TODO: only need to get atomic numbers once and cache it?
-    # is there a way to check atom types are mapped correctly?
-    atom_type_np = lmp.numpy.extract_atom("type")
-    masses = lmp.numpy.extract_atom("mass")
-    atomic_mass_arr = masses[atom_type_np]
-    atomic_numbers = lookup_atomic_number_by_mass(atomic_mass_arr)
-    boxlo, boxhi, xy, yz, xz, periodicity, box_change = lmp.extract_box()
-    cell = cell_from_lammps_box(boxlo, boxhi, xy, yz, xz)
-    atomic_data = atomic_data_from_lammps_data(
-        x, atomic_numbers, nlocal, cell, periodicity, lmp._task_name
-    )
-    results = lmp._predictor.predict(atomic_data)
-    assert "forces" in results, "forces must be in results"
-    f[:] = results["forces"].cpu().numpy()[:]
-    lmp.fix_external_set_energy_global(FIX_EXT_ID, results["energy"].item())
-
-    # during NPT for example, box_change should be set to 1 by lammps to allow the cell to change
-    if box_change:
-        # stress is defined as virial/volume in lammps
-        assert "stress" in results, "stress must be in results to compute virial"
-        volume = torch.det(cell).abs().item()
-        v = (results["stress"].cpu() * volume)[0]
-        # virials need to be in this order: xx, yy, zz, xy, xz, yz. https://docs.lammps.org/Library_utility.html#_CPPv437lammps_fix_external_set_virial_globalPvPKcPd
-        virial_arr = [v[0], v[4], v[8], v[1], v[2], v[5]]
-        lmp.fix_external_set_virial_global(FIX_EXT_ID, virial_arr)
+class FixExternalCallback:
+    def __init__(self, charge: int = 0, spin: int = 0):
+        self.charge = charge
+        self.spin = spin
+
+    def __call__(self, lmp, ntimestep, nlocal, tag, x, f):
+        # force copy here, otherwise we can accident modify the original array in lammps
+        # TODO: only need to get atomic numbers once and cache it?
+        # is there a way to check atom types are mapped correctly?
+        atom_type_np = lmp.numpy.extract_atom("type")
+        masses = lmp.numpy.extract_atom("mass")
+        atomic_mass_arr = masses[atom_type_np]
+        atomic_numbers = lookup_atomic_number_by_mass(atomic_mass_arr)
+        boxlo, boxhi, xy, yz, xz, periodicity, box_change = lmp.extract_box()
+        cell = cell_from_lammps_box(boxlo, boxhi, xy, yz, xz)
+
+        x_wrapped = wrap_positions(
+            x, cell=cell.squeeze().numpy(), pbc=periodicity, eps=0
+        )
+
+        atomic_data = atomic_data_from_lammps_data(
+            x_wrapped,
+            atomic_numbers,
+            nlocal,
+            cell,
+            periodicity,
+            lmp._task_name,
+            charge=self.charge,
+            spin=self.spin,
+        )
+        results = lmp._predictor.predict(atomic_data)
+        assert "forces" in results, "forces must be in results"
+        f[:] = results["forces"].cpu().numpy()[:]
+        lmp.fix_external_set_energy_global(FIX_EXT_ID, results["energy"].item())
+
+        # during NPT for example, box_change should be set to 1 by lammps to allow the cell to change
+        if box_change:
+            # stress is defined as -virial/volume in lammps
+            assert "stress" in results, "stress must be in results to compute virial"
+            volume = torch.det(cell).abs().item()
+            v = (-results["stress"].detach().cpu() * volume)[0].tolist()
+            # virials need to be in this order: xx, yy, zz, xy, xz, yz. https://docs.lammps.org/Library_utility.html#_CPPv437lammps_fix_external_set_virial_globalPvPKcPd
+            virial_arr = [v[0], v[4], v[8], v[1], v[2], v[5]]
+            lmp.fix_external_set_virial_global(FIX_EXT_ID, virial_arr)
 
 
 def run_lammps_with_fairchem(
-    predictor: MLIPPredictUnitProtocol, lammps_input_path: str, task_name: str
+    predictor: MLIPPredictUnitProtocol,
+    lammps_input_path: str,
+    task_name: str,
+    charge: int = 0,
+    spin: int = 0,
 ):
     machine = None
     if "LAMMPS_MACHINE_NAME" in os.environ:
         machine = os.environ["LAMMPS_MACHINE_NAME"]
     lmp = lammps(name=machine, cmdargs=["-nocite", "-log", "none", "-echo", "screen"])
     lmp._predictor = predictor
     lmp._task_name = task_name
-    run_cmds = []
+    # run_cmds = []
     with open(lammps_input_path) as f:
         input_script = f.read()
         check_input_script(input_script)
         script, run_cmds = separate_run_commands(input_script)
         logging.info(f"Running input script: {input_script}")
         lmp.commands_list(script)
         lmp.command(FIX_EXTERNAL_CMD)
+        fix_external_call_back = FixExternalCallback(charge=charge, spin=spin)
         lmp.set_fix_external_callback(FIX_EXT_ID, fix_external_call_back, lmp)
         lmp.commands_list(run_cmds)
     return lmp
@@ -203,7 +233,13 @@ def run_lammps_with_fairchem(
 )
 def main(cfg: DictConfig):
     predict_unit = hydra.utils.instantiate(cfg.predict_unit)
-    lmp = run_lammps_with_fairchem(predict_unit, cfg.lmp_in, cfg.task_name)
+    lmp = run_lammps_with_fairchem(
+        predict_unit,
+        cfg.lmp_in,
+        cfg.task_name,
+        cfg.charge,
+        cfg.spin,
+    )
     # this is required to cleanup the predictor
     del lmp._predictor
 

src/fairchem/lammps/lammps_fc_config.yaml

@@ -30,3 +30,5 @@ parallel_predict_unit:
 predict_unit: ${local_predict_unit}
 lmp_in: "lammps_in_example.file"
 task_name: "omol"
+charge: 0
+spin: 0

tests/lammps/lammps_npt.file

@@ -0,0 +1,19 @@
+units           metal              # Use metal units (Angstroms, eV, ps)
+atom_style      atomic             # Atoms have a single type and position
+lattice fcc 3.567
+boundary p p p
+
+region          simbox block 0 2 0 2 0 2
+create_box      1 simbox
+create_atoms    1 region simbox
+mass 1 12.011
+
+velocity        all create 300.0 12345 dist gaussian # Set initial velocities at 300 K
+
+timestep        0.001 # 1 fs
+# Use NPT (isotropic) thermostat+barostat: target temp 300 K, target pressure 0 bar
+# fix npt syntax: fix ID group-ID npt temp Tstart Tstop Tdamp iso Pstart Pstop Pdamp
+fix 1 all npt temp 300.0 300.0 0.1 iso 0.0 0.0 1.0
+thermo_style custom step temp pe ke etotal press vol
+thermo 1
+run 100

tests/lammps/test_ase_vs_lammps.py

@@ -5,12 +5,13 @@
 from ase import units
 from ase.build import bulk
 from ase.md.langevin import Langevin
+from ase.md.nose_hoover_chain import IsotropicMTKNPT
 from ase.md.velocitydistribution import MaxwellBoltzmannDistribution
 from ase.md.verlet import VelocityVerlet
-from fairchem.lammps.lammps_fc import run_lammps_with_fairchem
 
 from fairchem.core import FAIRChemCalculator
 from fairchem.core.calculate import pretrained_mlip
+from fairchem.lammps.lammps_fc import run_lammps_with_fairchem
 
 
 def run_ase_langevin():
@@ -74,6 +75,62 @@ def print_thermo(a=atoms):
     return atoms.get_kinetic_energy(), atoms.get_potential_energy()
 
 
+def run_ase_npt():
+    """Run ASE NPT-like using a Berendsen barostat approximation via NPT wrapper.
+
+    ASE doesn't provide a direct NPT integrator in the core; here we mimic
+    an NPT run by coupling to a thermostat and using the `Parrinello-Rahman`
+    style barostat if available in user's setup. For portability in tests we
+    instead run VelocityVerlet with a simple rescaling of the cell using the
+    `ase.constraints` is out of scope — this is a lightweight smoke test to
+    exercise the predictor through an NPT LAMMPS run for comparison.
+    """
+    atoms = bulk("C", "fcc", a=3.567, cubic=True)
+    atoms = atoms.repeat((2, 2, 2))
+    predictor = pretrained_mlip.get_predict_unit("uma-s-1p1", device="cuda")
+    atoms.calc = FAIRChemCalculator(predictor, task_name="omat")
+    initial_temperature_K = 300.0
+    np.random.seed(12345)
+    MaxwellBoltzmannDistribution(atoms, temperature_K=initial_temperature_K)
+    # Use ASE's NPT integrator which couples Nose-Hoover thermostat and
+    # barostat (Parrinello-Rahman style) and updates the cell. We pick
+    # thermostat/barostat time constants that map to LAMMPS fix npt's
+    # Tdamp/Pdamp (units: ps here for LAMMPS). ASE's API expects time in
+    # fs via ase.units, so use 0.1 ps = 100 fs as the thermostat time constant.
+    tdamp = 0.1  # ps (thermostat damping time for LAMMPS mapping)
+    pdamp = 1.0  # ps (barostat damping time for LAMMPS mapping)
+
+    # Convert ps -> fs for ASE NPT ttime/pfactor which expect time in fs units
+    tdamp_fs = tdamp * 1000.0 * units.fs
+    pdamp_fs = pdamp * 1000.0 * units.fs
+
+    # ASE NPT takes timestep in ASE units (seconds via units.fs) and temperature_K
+    # externalstress is pressure in eV/Å^3 or a scalar (here 0 means 0 pressure)
+    dyn = IsotropicMTKNPT(
+        atoms,
+        timestep=1.0 * units.fs,
+        temperature_K=300,
+        pressure_au=0.0 * units.bar,
+        tdamp=tdamp_fs,
+        pdamp=pdamp_fs,
+    )
+
+    def print_thermo(a=atoms):
+        ekin = a.get_kinetic_energy()
+        epot = a.get_potential_energy()
+        etot = ekin + epot
+        temp = ekin / (1.5 * units.kB) / len(a)
+        vol = a.get_volume()
+        print(
+            f"Step: {dyn.get_number_of_steps()}, Temp: {temp:.2f} K, "
+            f"Ekin: {ekin:.4f} eV, Epot: {epot:.4f} eV, Etot: {etot:.4f} eV, Vol: {vol:.4f} Å^3"
+        )
+
+    dyn.attach(print_thermo, interval=1)
+    dyn.run(100)
+    return atoms.get_kinetic_energy(), atoms.get_potential_energy()
+
+
 def run_lammps(input_file):
     predictor = pretrained_mlip.get_predict_unit("uma-s-1p1", device="cuda")
     lmp = run_lammps_with_fairchem(predictor, input_file, "omat")
@@ -88,6 +145,14 @@ def test_ase_vs_lammps_nve():
     assert np.isclose(ase_pot, lammps_pot, rtol=0.1)
 
 
+@pytest.mark.gpu()
+def test_ase_vs_lammps_npt():
+    ase_kinetic, ase_pot = run_ase_npt()
+    lammps_kinetic, lammps_pot = run_lammps("tests/lammps/lammps_npt.file")
+    assert np.isclose(ase_kinetic, lammps_kinetic, rtol=0.5)
+    assert np.isclose(ase_pot, lammps_pot, rtol=0.5)
+
+
 @pytest.mark.xfail(
     reason="This is more demo purposes, need to configure the right parameters for ASE langevin to match lammps"
 )