Large-scale first-principles molecular dynamics simulations: application to the microsolvation of biologically-relevant ions in aqueous clusters

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First-principles molecular dynamics simulations are employed to investigate the solvation structure of the biologically-relevant paradigm guanidinium-ion in water clusters, Gdn+(H2O)n. In these simulations, the intermolecular interactions are evaluated along the trajectories with the self-consistent-charge density-functional tight-binding (SCC-DFTB) model with the on-site third-order correction and modified effective Coulomb interaction, an approximate model of density-functional theory. For Gdn+(H2O)21, the simulated probability distribution of the ion-to-water cluster centre-of-mass distances and water angular coordinates suggests that Gdn+ is primarily localised at the surface of the water cluster, with the surface of the ion relatively devoid of water molecules. Estimates of the computational resources required for first-principles molecular dynamics simulations of larger Gdn+(H2O)n (n≤100) indicate that, with a modern supercomputer, such simulations can readily be performed in a matter of days.