Bremen Center for Computational Materials Science, Universität Bremen, Am Fallturm 1, 28359 Bremen, Germany.
J Chem Phys. 2011 Jul 21;135(3):034701. doi: 10.1063/1.3609768.
In experiments, the growth rate of ice from supercooled water is seen to increase with the degree of supercooling, that is, the lower the temperature, the faster the crystallization takes place. In molecular dynamics simulations of the freezing process, however, the temperature is usually kept constant by means of a thermostat that artificially removes the heat released during the crystallization by scaling the velocities of the particles. This direct removal of energy from the system replaces a more realistic heat-conduction mechanism and is believed to be responsible for the curious observation that the thermostatted ice growth proceeds fastest near the melting point and more slowly at lower temperatures, which is exactly opposite to the experimental findings [M. A. Carignano, P. B. Shepson, and I. Szleifer, Mol. Phys. 103, 2957 (2005)]. This trend is explained by the diffusion and the reorientation of molecules in the liquid becoming the rate-determining steps for the crystal growth, both of which are slower at low temperatures. Yet, for a different set of simulations, a kinetic behavior analogous to the experimental finding has been reported [H. Nada and Y. Furukawa, J. Crystal Growth 283, 242 (2005)]. To clarify this apparent contradiction, we perform relatively long simulations of the TIP4P/Ice model in an extended range of temperatures. The temperature dependence of the thermostatted ice growth is seen to be more complex than was previously reported: The crystallization process is very slow close to the melting point at 270 K, where the thermodynamic driving force for the phase transition is weak. On lowering the temperature, the growth rate initially increases, but displays a maximum near 260 K. At even lower temperatures, the freezing process slows down again due to the reduced diffusivity in the liquid. The velocity of the thermostatted melting process, in contrast, shows a monotonic increase upon raising the temperature beyond the normal melting point. In this case, the effects of the increasing thermodynamic driving force and the faster diffusion at higher temperatures reinforce each other. In the context of this study, we also report data for the diffusion coefficient as a function of temperature for the water models TIP4P/Ice and TIP4P/2005.
在实验中，人们观察到过冷水中冰的生长速率随着过冷度的增加而增加，也就是说，温度越低，结晶过程发生得越快。然而，在冻结过程的分子动力学模拟中，通常通过一种恒温器来保持温度恒定，恒温器通过对粒子的速度进行缩放来人为地去除结晶过程中释放的热量。这种直接从系统中去除能量的方法取代了更现实的热传导机制，据信这是导致一个奇特观察结果的原因，即在恒温条件下，冰的生长在熔点附近最快，而在较低温度下则较慢，这与实验结果完全相反[M.A.Carignano、P.B.Shepson 和 I.Szleifer，Mol.Phys.103,2957（2005）]。这种趋势可以通过扩散和液体中分子的重新取向来解释，它们在晶体生长中成为决定速率的步骤，而这两者在低温下都较慢。然而，对于另一组模拟，已经报道了类似于实验结果的动力学行为[H.Nada 和 Y.Furukawa，J.Crystal Growth 283,242（2005）]。为了澄清这种明显的矛盾，我们在扩展的温度范围内对 TIP4P/Ice 模型进行了相对较长时间的模拟。结果表明，恒温冰生长的温度依赖性比以前报道的更为复杂：在 270 K 的熔点附近，结晶过程非常缓慢，因为相变的热力学驱动力较弱。随着温度的降低，生长速率最初会增加，但在 260 K 附近会出现最大值。在更低的温度下，由于液体中的扩散率降低，冻结过程再次减慢。相比之下，恒温熔化过程的速度在正常熔点以上升高时会单调增加。在这种情况下，不断增加的热力学驱动力和较高温度下更快的扩散的影响相互加强。在本研究的背景下，我们还报告了 TIP4P/Ice 和 TIP4P/2005 水模型的温度相关扩散系数的数据。
