Coupled density functional/molecular mechanics Monte Carlo simulations of ions in water. The bromide ion

A recently developed NPT Monte Carlo method using density functional theory/molecular mechanics potentials is applied to study ion hydration. Test calculations for Na+ and Cl− are in agreement with experimental and classical simulation results. In the case of Br− the DFT/MM atom-atom radial distribution function is in better agreement with the experimental bromide-oxygen distance than that obtained in the classical simulation. Both models predict a preferred linear BrH0 arrangement. Fluctuations of the bromide ion electronic cloud are illustrated through the calculation of the instantaneous dipole moment of the ion.