GROMACS: MINFF mineral force field (structure → atom types/charges → MD) #241

Goal

Support the MINFF force field for minerals in ECCE's GROMACS workflow: clay minerals, oxides, hydroxides and zeolites, with Si, Al, Fe, Mg, Ca, Ti, Li and F. A user should be able to take a mineral unit cell, build a simulation cell from it, have MINFF atom types and charges assigned, and run energy minimisation, equilibration and dynamics, all from ECCE.

This builds on GROMACS activation via the MD workflow (#106, in progress on wip/gromacs-md).

What MINFF consists of

• Parameters as a GROMACS force-field directory (min.ff):

  • a general set (GMINFF) and mineral-specific sets (TMINFF, over 30 minerals);
  • each set in four angle force constants (0, 250, 500 and 1500 kJ/mol/rad²);
  • oxygen Lennard-Jones parameters taken from the OPC3 water model by default.

• Atom-type and charge assignment, which is structure-dependent:

  • types come from nearest neighbours;
  • oxygen charges are computed per system (charge smearing after Lammers et al. 2017), e.g. around isomorphic substitutions and edge groups.

  This is implemented in atomipy (Python, PyPI, MIT licence) and in the MATLAB/Octave atom library. ECCE therefore has to run the assignment rather than only ship parameter files.

• Example systems (unit cells, conf.mdp, topol.top) in the minff repository.

Proposed workflow in ECCE

1. Structure:
   • import a mineral unit cell (.pdb, .gro, CIF), with the cell vectors;
   • replicate it to a supercell (n_a × n_b × n_c);
   • optionally substitute atoms with a minimum separation (e.g. Al→Mg), as atomipy does.
2. Force-field choice:
   • general or mineral-specific MINFF (mineral list taken from min.ff);
   • angle force constant;
   • water model.
3. Assignment:
   • ECCE runs atomipy;
   • it writes the mineral .itp, the .gro and a topol.top that includes min.ff;
   • it reports the total charge and any atoms left untyped.
4. Show the result:
   • atoms coloured by MINFF type;
   • charges available as a property.
5. Simulation with the GROMACS MD tasks (GROMACS: the scope decision that gates the rest (topology and force field) #106): EM → NVT/NPT → MD, with .mdp presets derived from the minff example conf.mdp.
6. Later:
   • solvation and counter-ions (hydrated clays, e.g. Na-montmorillonite);
   • analysis (MD analysis for electrolytes: RDF, coordination numbers, residence times, diffusion, conductivity, transference number #180).

Design rules

• The topology builder is a wx-free component with a command-line entry point and tests. The ECCE windows only collect the choices and display results.
• Tests: kaolinite and montmorillonite from the minff Systems folder are built with ECCE's builder. They are checked against topologies produced by atomipy run directly (atom types, charges, total charge), and then go through grompp and a short EM with real GROMACS.
• Parameter files: min.ff is fetched from the minff repository at build/test time and not copied into this repository. It can be bundled once the minff repository carries a licence.
• Platforms: Linux and macOS (Homebrew gromacs).

Open questions for GROMACS/MINFF users

• Which minerals and systems should come first?
• Is the general set with k = 500 the right default?
• Should ECCE offer CLAYFF (also supported by atomipy) alongside MINFF?
• Which .mdp settings should the presets use: cut-offs, PME, thermostat/barostat, semi-isotropic pressure coupling for clay layers?