液態水的定向自相關函數：SPC/E模型

Abstract

在此論文，我應用分子動力學(MD)模擬液態水SPC/E模型，計算其定向自相關函數(OCF)，並且運用相對應的擬合函數來討論水分子在不同時間尺度下的轉動。此外，藉由Voronoi多面體(VP)分析，我們研究氫鍵所產生的局部結構對定向自相關函數的影響。 在短時間尺度下，定向自相關函數應該是一高斯衰減，此高斯衰減可用瞬間正則模(INM)理論或角速度自相關函數(AVAF)所得到的功率頻譜來驗證。在長時間尺度下，根據Debye理論，定向自相關函數的行為應該是一個單一指數函數衰減，因此運用線性擬合函數來測試OCF的對數函數在長時間是否會遵守Debye理論。在中間時間尺度，扣除掉在長時間尺度下的單一指數衰減，所剩餘的OCFs可由一猜測的函數完全擬合，而此擬合函數可用一個由冪次函數乘上一個高斯函數來描述。 我的結果指出，局部結構內的氫鍵對OCFs在短時間尺度下扮演重要的角色，但在長時間尺度下，沒有顯著的效應。

In this thesis, I apply molecular dynamics (MD) to simulate the SPC/E model of liquid water and calculate the orientational correlation functions (OCFs) of the water model. The simulated OCFs in different time scales are described by various fitting functions and, then, we discuss the orientational motions in liquid water through these fitting functions. Besides, by Voronoi polyhedra (VP) analysis for the local structures in the simulated liquid, we study the H-bond effects on the OCFs of this water model. In the short-time regime, the OCFs should be a Gaussian decay, which can be verified by the instantaneous-normal-mode (INM) theory or the power spectra of the angular velocity autocorrelation functions (AVAFs). In the long-time regime, according to the Debye theory, the behavior of the OCFs should be a single exponential decay and a linear fitting function is used to test whether the long-time behavior of the logarithm of the OCFs obeys the Debye theory. In the intermediate-time regime, by subtracted out the single-exponential decay in the long-time regime, the residual OCFs can be well fit by a surmised function, which is a power-law function multiplied by a Gaussian function. My results indicate that the H-bonds of local structures play an important role on the OCFs in the short-time regime but have no significant effects in the long-time regime.