基于曲线质心的水的路径积分动力学
Path-integral dynamics of water using curvilinear centroids
摘要：
我们开发了一个类似于质心分子动力学（CMD）的水的路径积分动力学方法，除了质心是曲线的平均值，而不是carbohydrate，珠坐标。曲线坐标仅在计算平均力的势时才明确使用，平均力的分量用腕力的“准质心”（因为它们接近腕力的质心而被称为“准质心”）重新表示。笛卡尔运动方程是通过对量子玻尔兹曼分布进行小的近似而得到的。在150-600 K温度范围内，对各种水模型的红外光谱进行了模拟，结果表明：对于二维OH键模型，准质心分子动力学（QCMD）谱接近于精确量子谱，几乎位于松原动力学谱之上，对于气相水，QCMD谱接近于精确量子谱;对于液态水和冰（使用q-TIP 4P/F表面），QCMD谱在600 K接近CMD谱，并且与恒温环聚合物分子动力学和近似量子计算在300和150 K的结果一致。QCMD光谱没有显示出CMD“曲率问题”（错误的红移和加宽）的迹象。在液体和冰的模拟中，平均力的潜力是通过将绝热CMD算法推广到曲线坐标来评估的;需要采取绝热分离的全部限制，这使得QCMD计算在300 K时比部分绝热CMD贵8倍，在150 K时贵32倍（并且强度在该温度下可能仍然不收敛）。QCMD方法可能是generalisable许多其他系统，提供集体珠坐标可以确定，产生紧凑的平均场环聚合物分布。
英文摘要：
We develop a path-integral dynamics method for water that resembles centroid molecular dynamics (CMD), except that the centroids are averages of curvilinear, rather than cartesian, bead coordinates. The curvilinear coordinates are used explicitly only when computing the potential of mean force, the components of which are re-expressed in terms of cartesian 'quasi-centroids' (so-called because they are close to the cartesian centroids). Cartesian equations of motion are obtained by making small approximations to the quantum Boltzmann distribution. Simulations of the infrared spectra of various water models over 150-600 K show these approximations to be justified: for a two-dimensional OH-bond model, the quasi-centroid molecular dynamics (QCMD) spectra lie close to the exact quantum spectra, and almost on top of the Matsubara dynamics spectra; for gas-phase water, the QCMD spectra are close to the exact quantum spectra; for liquid water and ice (using the q-TIP4P/F surface), the QCMD spectra are close to the CMD spectra at 600 K, and line up with the results of thermostatted ring-polymer molecular dynamics and approximate quantum calculations at 300 and 150 K. The QCMD spectra show no sign of the CMD 'curvature problem' (of erroneous red shifts and broadening). In the liquid and ice simulations, the potential of mean force was evaluated on the fly by generalising an adiabatic CMD algorithm to curvilinear coordinates; the full limit of adiabatic separation needed to be taken, which made the QCMD calculations 8 times more expensive than partially adiabatic CMD at 300 K, and 32 times at 150 K (and the intensities may still not be converged at this temperature). The QCMD method is probably generalisable to many other systems, provided collective bead-coordinates can be identified that yield compact mean-field ring-polymer distributions.
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