TEST OF MODE COUPLING THEORY FOR A SUPERCOOLED LIQUID OF DIATOMIC MOLECULES. I. TRANSLATIONAL DEGREES OF FREEDOM
TEST OF MODE COUPLING THEORY FOR A SUPERCOOLED LIQUID OF DIATOMIC MOLECULES. I. TRANSLATIONAL DEGREES OF FREEDOM
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双原子分子过冷液体的模式耦合理论测试。
DOI：
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发表时间：
1997
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通讯作者：
R. Schilling
中科院分区：
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作者：
Stefan Kammerer;W. Kob;R. Schilling
对刚性双原子分子的过冷液体进行了分子动力学模拟。确定了分子质量中心随时间变化的自密度和集体密度相关系数，并与简单液体的理想模式耦合理论（MCT）的预测结果进行了比较。这在实空间和动量空间中都是可行的。其中一个主要结果是存在一个唯一的转变温度${T}_{c}，在这个温度下动力学从遍历行为过渡到拟非遍历行为。${T}_{c}$的值与前面发现的方向动力学的值在误差条内一致。在MCT的第一标度律区域（也称为ensuremath{eta}区域）中，我们发现ensuremath{eta}区域后期的相关系数可以用von Schweidler定律很好地拟合。虽然我们没有观察到MCT预测的早期确保{eta}松弛状态的纯粹形式的临界衰变，但我们的松弛曲线表明这种衰变确实存在。在第一个标度体系中，只有考虑到对渐近律的下一阶修正，才会出现理想MCT内的一致描述。对于${q=q}_{mathrm{max}}（静态结构因子$S(q)中主峰的位置）来说，这种修正几乎可以忽略不计，但对于${q=q}_{mathrm{min}}（其第一个最小值的位置）来说，这种修正变得很重要。第二标度定律，即时间-温度叠加原理，对于自密度和集体密度相关器以及不同的$q值都适用。ensuremath{alpha}-松弛时间${ensuremath{au}}_{q}^{(s)}$和${ensuremath{au}}_{q}$遵循一个幂律在$Tensuremath{-}{T}_{c}$中超过二到三十年。相应的指数ensuremath{gamma}实际上是$q$独立的，大约在2.55左右。该值与MCT根据von Schweidler指数预测的值一致，但与前人研究的方向相关器${C}_{1}^{(s)}(t)$和${C}_{1}(t) $对应的指数ensuremath{gamma}ensuremath{approx}1.6存在差异。
A molecular-dynamics simulation is performed for a supercooled liquid of rigid diatomic molecules. The time-dependent self and collective density correlators of the molecular centers of mass are determined and compared with the predictions of the ideal mode coupling theory (MCT) for simple liquids. This is done in real as well as in momentum space. One of the main results is the existence of a unique transition temperature ${T}_{c},$ at which the dynamics crosses over from an ergodic to a quasinonergodic behavior. The value for ${T}_{c}$ agrees within the error bars with that found earlier for the orientational dynamics. In the first scaling law regime of MCT, also called the ensuremath{eta} regime, we find that the correlators in the late stage of the ensuremath{eta} regime can be fitted well by the von Schweidler law. Although we do not observe the critical decay predicted by MCT for the early ensuremath{eta}-relaxation regime in its pure form, our relaxation curves suggest that this decay is indeed present. In this first scaling regime, a consistent description within ideal MCT emerges only, if the next order correction to the asymptotic law is taken into account. This correction is almost negligible for ${q=q}_{mathrm{max}},$ the position of the main peak in the static structure factor $S(q),$ but becomes important for ${q=q}_{mathrm{min}},$ the position of its first minimum. The second scaling law, i.e., the time-temperature superposition principle, holds reasonably well for the self and collective density correlators and different values for $q.$ The ensuremath{alpha}-relaxation times ${ensuremath{ au}}_{q}^{(s)}$ and ${ensuremath{ au}}_{q}$ follow a power law in $Tensuremath{-}{T}_{c}$ over two to three decades. The corresponding exponent ensuremath{gamma} is practically $q$ independent and is around 2.55. This value is in agreement with the one predicted by MCT from the value of the von Schweidler exponent but at variance with the corresponding exponent ensuremath{gamma}ensuremath{approx}1.6 obtained for the orientational correlators ${C}_{1}^{(s)}(t)$ and ${C}_{1}(t),$ studied in a previous paper.