Equipartition and the temperature of maximum density of TIP4/2005 water

TL;DR

This study investigates how simulation parameters like time-step size affect the accuracy of water properties in TIP4P/2005 water models, emphasizing the importance of short time-steps for reliable biomolecular simulations.

Contribution

It demonstrates that short simulation time-steps are crucial for accurate water density and phase behavior, highlighting the impact of simulation settings on model reliability.

Findings

Short time-steps (≤0.5 fs) yield accurate density and TMD values.

Longer time-steps cause shifts in the temperature of maximum density.

Enhancing dispersion interactions degrades the liquid-vapor phase description.

Abstract

We simulate TIP4P/2005 water in the temperature range of 257 K to 318 K with time-steps $\delta =$ 0.25, 0.50, 2.00, and 4.00 fs. The density-temperature behavior obtained using 0.25 or 0.50 fs are in excellent agreement with each other but differ from those obtained using time-steps that have been shown earlier to lead to a breakdown of equipartition. The temperature of maximum density (TMD) is 277.15 K with $\delta t = 0.25\;\mathrm{or}\; 0.50$ fs, but is shifted to progressively lower values for longer time-steps, a trend that holds for different thermostat/barostat combinations. Enhancing the water-water dispersion interaction, as has been recommended for simulating disordered proteins in TIP4P/2005, degrades the description of the liquid-vapor phase envelope. A key takeaway from this study is that using sufficiently short time-steps ($\leq 0.5$ fs) to preserve equipartition is essential for obtaining meaningful liquid water properties and for producing reliable data to parametrize biomolecular simulation models, as correct-ensemble sampling is fundamental to ensure reproducibility across codes and simulation alogrithms.