ModuleNotFoundError: No module named 'mliap_unified_couple' when using MACE with MLIAP in LAMMPS Python

[stargolike]

When attempting to run a simulation using MACE interatomic potential via LAMMPS Python interface with MLIAP support, I encounter a ModuleNotFoundError related to the mliap_unified_couple module. The error prevents loading the MLIAP module, causing the simulation to fail.

Steps to Reproduce

1. Code Setup
   Run the following Python script (attached below) to initialize LAMMPS, configure MACE potential via MLIAP, and attempt a simulation:

   from lammps import lammps
   import time
   
   # Initialize LAMMPS instance
   lmp = lammps()
   
   # Basic simulation settings
   lmp.command("units metal")                  
   lmp.command("boundary p p p")              
   lmp.command("atom_style atomic")           
   lmp.command("timestep 0.001")              
   
   # Activate MLIAP support
   import lammps.mliap
   lammps.mliap.activate_mliappy_kokkos(lmp)
   
   # Configure MACE potential
   lmp.command("pair_style mliap unified test.pt 0")
   lmp.command("read_data /root/calphy/examples/example_01/fe - bcc - solid - 100 - 0.0.data")  
   lmp.command("pair_coeff * * Fe")          
   
   # Set atomic mass
   lmp.command("mass 1 55.845")
   
   # Define custom variables
   lmp.command("variable mvol equal vol")     
   lmp.command("variable mlx equal lx")       
   lmp.command("variable mly equal ly")       
   lmp.command("variable mlz equal lz")       
   lmp.command("variable mpress equal press") 
   
   # Initialize velocities
   random_seed = int(time.time())
   lmp.command(f"velocity all create 100.0 {random_seed} dist gaussian")  
   
   # Configure NPT ensemble
   lmp.command("fix nh1 all npt temp 100.0 100.0 0.1 iso 0.0 0.0 0.1")
   
   # Thermodynamic output
   lmp.command("thermo_style custom step temp press density etotal pe vol lx ly lz")  
   lmp.command("thermo 100")                  
   lmp.command("thermo_modify flush yes")     
   
   # Save thermo data
   lmp.command("fix thermo all print 100 '${step} ${temp} ${press} ${density} ${etotal} ${pe} ${vol} ${lx} ${ly} ${lz}' file thermo_output.txt screen no")
   
   # Equilibration
   print("Starting equilibration phase...")
   lmp.command("run 5000")  
   
   # Reset for production
   lmp.command("reset_timestep 0")
   lmp.command("thermo_reset")
   
   # Production run
   print("Starting production simulation...")
   lmp.command("run 10000")  
   
   print("Simulation completed successfully!")

2. Error Output
   Upon execution, the following error occurs:

   LAMMPS (12 Jun 2025 - Development - patch_12Jun2025 - 510 - g6795a2a066)
   OMP_NUM_THREADS environment is not set. Defaulting to 1 thread.
     using 1 OpenMP thread(s) per MPI task
   ModuleNotFoundError: No module named 'mliap_unified_couple'
   ERROR: Loading mliappy unified module failure. (src/ML - IAP/mliap_unified.cpp:210)
   Last input line: pair_style mliap unified test.pt 0
   Traceback (most recent call last):
     File "/root/test.py", line 18, in <module>
       lmp.command("pair_style mliap unified test.pt 0")
     File "/opt/mamba/lib/python3.10/site-packages/lammps/core.py", line 833, in command
       with ExceptionCheck(self):
     File "/opt/mamba/lib/python3.10/site-packages/lammps/core.py", line 59, in __exit__
       raise self.lmp._lammps_exception
   Exception: ERROR: Loading mliappy unified module failure. (src/ML - IAP/mliap_unified.cpp:210)
   Last input line: pair_style mliap unified test.pt 0
   Total wall time: 0:00:03
   

Expected Behavior
The simulation should initialize MLIAP with the MACE potential, run the equilibration and production phases, and output thermodynamic data without module - loading errors.

Additional Context

• The MACE model file test.pt is properly converted for LAMMPS MLIAP usage (previously functional with older setups, but issues arose after environment updates).
• Tried re - compiling LAMMPS with MLIAP and Kokkos support, but the error persists.
• Suspect a mismatch between LAMMPS Python interface, MLIAP module, and Kokkos activation (activate_mliappy_kokkos).

Looking for guidance on resolving the mliap_unified_couple module import failure and ensuring MLIAP with MACE works in the current LAMMPS/Python environment.

[stargolike]

and i try to use activate_mliappy , error will be
AttributeError: 'MLIAPDataPy' object has no attribute 'forward_exchange' when using MACE potential with LAMMPS ML-IAP

[ilyes319]

Did you make sure to install the latest develop branch of lammps?

[stargolike]

I find that's why. we need use special command to run lammps-mliap
lmp = lammps(cmdargs=["-k", "on", "g", "1", "-sf", "kk", "-pk", "kokkos", "newton", "on", "neigh", "half"])

[bernstei]

Yup, there are several requirements. See also #1056. They're lammps design decisions, not really MACE, but they might still be worth noting in the MACE docs just to reduce confusion.