Intra-molecular coupling as a mechanism for a liquid-liquid phase transition

We study a model for water with a tunable intra-molecular interaction $J_{\sigma }$, using mean field theory and off-lattice Monte Carlo simulations. For all $J_{\sigma }\ge 0$, the model displays a temperature of maximum density.For a finite intra-molecular interaction $J_{\sigma }>0$,our calculations support the presence of a liquid-liquid phase transition with a possible liquid-liquid critical point for water, likely pre-empted by inevitable freezing. For J=0 the liquid-liquid critical point disappears at T=0.