Packmol Generate Mixture

packmol-generate-mixture

Use Packmol to generate an initial packed configuration for a molecular mixture.

Agent responsibilities (do these in order)

1. Collect inputs (ask if missing; do not guess):

   • component structure files (XYZ), one per species (e.g. species1.xyz, species2.xyz)
   • molecule counts for each species (e.g. species1: 100, species2: 650)
   • either target density (g/cm^3) or a fixed cubic box length (Å)
   • Packmol tolerance (Å)
   • output location: output directory + output filename prefix (system name)

2. Validate inputs:

   • confirm XYZ files exist and are readable
   • confirm the first line (atom count) matches the number of coordinate lines
   • if density-based box estimation is requested: confirm each molecule’s elemental composition can be inferred from the XYZ symbols

3. Decide box size:

   • If user provides box_length_A: use it.
   • Else compute box_length_A from density (see formula below).

4. Create a working folder at the requested output location:

   • copy the component XYZ files into it (or reference them with absolute paths)

5. Write Packmol input ${system_name}.inp:

   • one structure ... end structure block per component
   • all components share the same inside box 0 0 0 L L L

6. Run Packmol locally:

   • Prefer: uvx packmol -i ${system_name}.inp
   • If you need to force the source package: uvx --from packmol packmol -i ${system_name}.inp

7. Report results:

   • exact output paths (inp, xyz, log)
   • final box length (Å) and the parameters used (counts, density or fixed L, tolerance)
   • basic sanity checks (total molecules, total atoms)

8. (Optional) Post-process for LAMMPS

If the user plans to run LAMMPS (especially ReaxFF), they often need a LAMMPS data file with correct box bounds.

• If you convert XYZ -> LAMMPS data with dpdata, dpdata may write default box bounds (e.g., 0..100 Å).
• Fix the bounds to match the Packmol cubic box length using lammps-md-tools from PyPI:

uvx --from lammps-md-tools lammps-fix-box \
    --in  input.data \
    --out output.boxfix.data \
    --L 60.690 \
    --wrap

This rewrites xlo/xhi, ylo/yhi, zlo/zhi to 0..L, zeroes tilt factors, and optionally wraps atoms into the box.

What to ask the user (plain language)

If the user didn’t specify them, ask at minimum:

• Packing counts: how many molecules of each species? (e.g., species1=100, species2=650)
• Box definition: do you want to estimate a cubic box from a target density (g/cm^3), or do you want to provide a fixed cubic box length L (Å)?
• Tolerance: what Packmol tolerance (Å) should be used? (common starting point: 2.0 Å)
• Output location: which directory should receive the results, and what system name / filename prefix should be used?

If the user says “use defaults”, propose defaults:

• tolerance = 2.0 Å
• output dir: a packed/ subfolder under the folder containing the input XYZ
• (density) do not assume; ask for it, but you may suggest a starting value the user can confirm.

Input schema (recommended)

Example (replace with your own species/files):

system_name: mixture_pack
output_dir: /path/to/output/packed
# Choose ONE of the following:
density_g_cm3: 0.25
# box_length_A: 60.69

tolerance_A: 2.0
components:
  - name: species1
    structure_file: /path/to/species1.xyz
    number: 100
  - name: species2
    structure_file: /path/to/species2.xyz
    number: 650

Density → cubic box length (Å)

When density_g_cm3 is provided and box_length_A is not, estimate L from total mass:

• infer each molecule’s elemental composition from its XYZ symbols
• use standard atomic masses (g/mol)
• compute total molar mass of the whole configuration (g/mol)
• convert to mass per configuration: m_cfg = M_total / N_A (g)
• compute volume in cm^3: V_cm3 = m_cfg / density_g_cm3
• convert to Å^3: V_A3 = V_cm3 * 1e24
• cubic length: L_A = V_A3 ** (1/3)

This is an initial packing estimate (geometry construction), not an equilibrated density.

Output contract

The run should produce (within output_dir):

• ${system_name}.inp (Packmol input)
• ${system_name}.xyz (packed XYZ output; name may include _packed suffix)
• packmol.out (stdout log; capture with tee)

Limitations (be explicit)

• Packed XYZ has coordinates only; no topology, no force-field types, no LAMMPS data.
• Packing success ≠ physically valid structure; minimization/equilibration still required.