Singularity Free Research Articles

On decreasing the temperatureT, the correlationtime τ of supercooled water displays a dynamic crossover from non-Arrhenius dynamics (withT-dependent activationenergy) at high T to Arrhenius dynamics (with constant activation energy) at lowT. Simulations for water models show that this crossover occurs at thelocus of maximum isobaric specific heat in the pressure–temperature(P–T) plane. Results of simulations show also that at this locus there is a sharp change oflocal structure: more tetrahedral below the locus, and less tetrahedral above it.Furthermore, in water solutions with proteins or DNA, simulations show thatin correspondence with this locus there is a crossover in the dynamics of thebiomolecules, a phenomenon commonly known as the protein glass transition.To clarify the relation of the dynamic crossover with the thermodynamics ofwater, we study the dynamics of a cell model of water which can be tuned toexhibit: (1) a first-order phase transition line that separates the liquids of highand low densities at low temperatures; this phase transition line terminates ata liquid–liquid critical point (LLCP), from which departs the Widom lineTW(P), i.e. the line of maximum isobaric specific heat in theP–T plane; (2) the singularity-free (SF) scenario, under which the system exhibits water-likeanomalies but with no finite temperature liquid–liquid critical point.We find that the dynamic crossover is present in both the LLCP andthe SF cases. Moreover, on the basis of the study of the probabilitypB of forming a bond, we propose and verify a relation between dynamics and thermodynamicsthat is able to show how the crossover is a consequence of a local relaxation processassociated with breaking a bond and reorienting the molecule. We further find a distinctdifference in pressure dependence of the dynamic crossover between the LLCP and SFscenarios, which may help in resolving which of the scenarios correctly explains theanomalous behavior of water.