On the Use of Constraints in Shock Simulations of CHARMM TIP3P Water

We seek to understand and predict the response of biological membranes to mechanical shock. Semi-flexible and rigid models are commonly used to model water in such systems under equilibrium conditions, with relatively minor effects on many equilibrium thermodynamic properties. Two important exceptions are the heat capacity and the Grüneisen parameter; both of which manifest strong quantum mechanical behaviors.

The nascent thermodynamic state behind a shock is sensitive to both the isochoric heat capacity and the Grüneisen parameter; together, they largely determine the temperature increase across a shock discontinuity. The implications for studying the kinds of the low-barrier, thermally activated, molecular-scale processes that govern membrane stability seem obvious. However, both of these properties are also sensitive to constraints in the Hamiltonian. We exploit this here to determine which set of intramolecular constraints best reproduces experimental shock wave data.