Introduction 
    Water Potential
    MD simulations 
    Results 
    Conclusions
    References

Slides

Home
 
 
 
 

Team 2 

Xiaohai Li
Vamsi Akkineni
Jianwei Wang 

Fall, 2002 

Xiaohai Li, Vamsi Akkineni, Jianwei Wang

Introduction

     Recently one of the most challenging problems in the physics of liquid water is to study the phase behaviors of the supercooled water^[1]. It has been proposed that in addition to the known critical point C, a second critical point C' might possibly occur at the low temperature which separates the liquid water into two distinct liquid phase: a low-density liquid (LDL) phase at low pressures and a high-density liquid (HDL) at high pressure (Fig. 1). This liquid-liquid phase transition hypothesis is difficult to prove experimentally because supercooled water freezes spontaneously below the homogeneous nucleation temperature T_H, and amorphous ice crystallizes above the crystallization temperature T_X. Freezing makes experimentation on the supercooled liquid state between T_H and T_X almost impossible.  Computer simulation becomes extremely useful in this case because nucleation does not occur on the timescale of computer simulation. 

     Computer simulations can be used to determine the position of the second critical point. It has been shown that there exists a line in the phase diagram along which the isothermal compressibility of the suepercooled liquid is a maximum. With the temperature decreases to T_C', the magnitude of the maximum diverges. Furthermore, compute simulations may also be used to probe the thermodynamic, structural, and transport properties of supercooled water. By running molecular dynamics simulation on different state points, we may track the phase transition. 

     However, water is particularly difficult to simulate because it is a molecular liquid and there is at present no universally agreed precise yet tractable intermolecular potential. But we can still get some useful information by comparing the simulation results with different intermolecular potentials. Among the most widely used intermolecular potential of water are ST2 model, TIP4P model and SPC model which will be discussed in more detail later. 

      In our work we choose SPC potential because of the two reasons. First, there are a lot of simulations published in the literatures which use ST2 potential or TIP4P potential ^[2-6], while the simulation based on SPC potential is relative rare. Second, SPC potential is easy to implement in the software (Cerius) we used. We are interested in the structure difference between two liquid phases. It usually takes more than one day  to run the simulation on one state point. So it is not realistic to get the second critical point because it needs a lot of state points. 

                                                                       Fig 1. The proposed phase diagram of water^[1]