State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China.
J Chem Phys. 2009 Nov 28;131(20):204503. doi: 10.1063/1.3265984.
A bridge function approximation is proposed to close the Ornstein-Zernike (OZ) integral equation for fluids with purely repulsive potentials. The performance of the bridge function approximation is then tested by applying the approximation to two kinds of repulsive potentials, namely, the square shoulder potential and the triangle shoulder potential. An extensive comparison between simulation and the OZ approach is performed over a wide density range for the fluid phase and several temperatures. It is found that the agreement between the two routes is excellent for not too low temperatures and satisfactory for extremely low temperatures. Then, this globally trustworthy OZ approach is used to investigate the possible existence or not of a liquid anomaly, i.e., a liquid-liquid phase transition at low temperatures and negative values of the thermal expansion coefficient in certain region of the phase diagram. While the existence of the liquid anomaly in the square shoulder potential has been previously predicted by a traditional first-order thermodynamic perturbation theory (TPT), the present investigation indicates that the liquid-liquid phase transition disappears in the OZ approach, so that its prediction by the first-order TPT is only an artifact originating from the low temperature inadequacy of the first-order TPT. However, the OZ approach indeed predicts negative thermal expansion coefficients. The present bridge function approximation, free of adjustable parameters, is suitable to be used within the context of a recently proposed nonhard sphere perturbation scheme.
提出了一种桥函数逼近方法，用于封闭具有纯排斥势的流体的奥恩斯坦-泽尔尼克（OZ）积分方程。然后，通过将该逼近方法应用于两种排斥势，即方肩势和三角肩势，来测试桥函数逼近方法的性能。在广泛的密度范围内和几个温度下，对流体相进行了模拟和 OZ 方法之间的广泛比较。结果表明，对于不太低的温度，两种方法之间的一致性非常好，对于极低的温度也令人满意。然后，使用这种全局可靠的 OZ 方法来研究在某些相图区域中是否存在液体异常，即低温下的液-液相转变和热膨胀系数为负值。虽然方肩势中的液体异常先前已被传统的一阶热力学微扰理论（TPT）预测，但本研究表明，OZ 方法中的液-液相转变消失了，因此一阶 TPT 的预测只是源于一阶 TPT 在低温下的不充分性的伪影。然而，OZ 方法确实预测了负的热膨胀系数。本研究中提出的无可调参数的桥函数逼近方法适用于最近提出的非硬球微扰方案的背景下。
