Water drops and atomic flops

I am currently stabilizing a reusable air-equivalent solvent in which to disperse radioactive colloids to study the changes to aerosol properties introduced by radiation effects using the GROMACS (van de Spoel, 2005) molecular dynamics package. The native all-atom force-field parameters for the TIP4P water model (Horn et al, 2004) was used, but had to be defined specifically for N2 and O2 using a recent optimization (Vujic, 2016). Structure input files for GROMACS were generated with Avogadro.

Figure 1: Femtosecond (10-15 s) timestep simulation of 781 N2 molecules, 209 O2 molecules, and 54 H2O molecules, randomly distributed in a 64 nm3 box.

After about 10 ps at 298 K, the volume came to equilibrium with 1 atm of externally applied pressure. The resulting trajectory was viewed in Avogadro, displayed in Fig 1. O2 and N2 (red and blue respectively) remain well-mixed, but the gap between each ion them where the double and triple bonds have broken; this is the result of my missing the massless “dummy” atom in the diatomic models that carries the covalent positive partial charge to bind the electronegative ions causing them to otherwise repel against the harmonic-oscillator restoration force! The water has dispersed into many sizes of colloids. One such instance of a larger agglomeration is present in the center, a droplet roughly 1 nm in diameter.