MolDStruct: benchmarking a hybrid Monte Carlo/Molecular Dynamics model for X-ray free-electron laser ionisation and fragmentation dynamics
Single Particle Imaging with intense X-ray free-electron laser pulses requires modelling of the resulting ionisation and Coulomb explosion dynamics of biomolecules to optimise experimental parameters and enable correct structural reconstruction, yet simulating the complete dynamics at protein scale is beyond the reach of quantum-mechanical methods. To address this, we developed \moldstruct, a hybrid Monte Carlo/Molecular Dynamics code built on GROMACS that couples high intense X-ray ionisation dynamics modelled through a Monte Carlo module with classical Molecular Dynamics for atomic propagation. Benchmarked against quantum mechanical calculations for di-alanine, MolDStruct agrees in fragmentation patterns above a mean charge per atom of $\bar{z} \approx 1.35$. Compared with Coulomb explosion imaging experimental data for 2-iodopyridine, simulated momentum distributions reproduce the experimental Newton plots in fragment direction and fall within the range of experimental absolute momenta, with a moderate overestimation. We further apply MolDStruct to Protein Explosion Imaging, a method that classifies molecular structures from explosion ion maps recorded on a detector, demonstrating via dimensionality reduction that conformers of the 16-residue peptide $\mathrm{Ala}_16$ and five chromophore-labelled ubiquitin mutants are distinguishable. These results establish MolDStruct as a practical tool for simulating radiation damage and Coulomb explosion dynamics of biomolecules at protein scale, where quantum-mechanical methods are computationally infeasible.