甲烷-矽烷分子二聚體之量子化學勢能計算及 甲烷流體之分子動力學模擬

Abstract

我們使用HF、MP2、DFT三種方法進行甲烷-矽烷間12個方位的分子作用力計算，所有的計算都已加入BSSE進行修正。HF的計算主要會產生排斥的作用力，且經過量子化學計算後得知基底收斂在cc-pVQZ。MP2的計算則可產生整條的凡得瓦爾力曲線，且基底的影響相當明顯，計算可得知收斂在0.05kcal/mol。此外，對於6個非對稱構型，改變中心原子的位置計算會產生不同的HF及MP2計算結果。我們使用大範圍exchange-correlation functionals配對的DFT計算，並與MP2的計算結果加以比較。 將甲烷-甲烷及矽烷-矽烷的ab initio檔案帶入混合理論(Mixing rule)進行計算並與甲烷-矽烷的結果加以比較，發現E、F、I、J構型可產生良好的計算結果，而其他方位則需要加以考慮靜電力的效應加以修正。 我們將量子化學計算出的甲烷位能曲線利用4 sites Lennard-Jones potentials 擬合出鍵距與束縛能參數，帶入牛頓方程式進行分子動力學模擬，對於甲烷液體的平衡及動態性質，與實驗結果比較已相當精確且逼近實驗值的誤差下標，這可說明使用ab initio所建構之力場進行分子動力學模擬是具有相當的精確性。利用4 sites位能模型去模擬甲烷實驗三相圖的氣化曲線與溶化曲線，可幫助我們了解不同相態下的平衡及動態性質。

We have calculated the interaction potentials of the 12 methane-silane conformations using the Hartree-Fock (HF)self-consistent theory, the correlation-corrected second-order Møller-Plesset (MP2) perturbation theory, and the density functional theory (DFT) with the basis set superposition error (BSSE) corrected. The HF calculation yield repulsive potentials and converged at the cc-pVQZ basis set after quantitatively calculation. The MP2 calculation yield whole van der waals potential curves. The basis set effect are observable, and the calculation converged in 0.05kca/mol. Moreover, change the C-Si position of the six non-symmetric conformers will yield different HF and MP2 calculations. The DFT calculations generate a wide range of interaction patterns, and compare with MP2 results. We use Mixing rule to calculation the methane-silane interaction from the methane-methane and silane-silane ab initio data, and found that the result have good agreement with the methane-silane ab initio data in E、F、I、J conformers. Others are corrigible with the electrostatic. Next, we use 4 sites-fitting method to fit the results of quantum chemistry calculation. We can get the force parameters of bond lengths and binding energies and then input them to solve Newton's equations. To simulate the equilibrium properties and dynamics properties of methane liquid, we perform NVT ensemble molecular dynamics simulations. We compare the results with experiments from different research groups, and found that our PES is capable of reproducing the experimental data within the error bars. It demonstrates that quantum chemistry calculated intermolecular interaction is very good which can accurately yield the molecular dynamic simulation results. Beside we simulate the equilibrium properties and dynamic properties along the vapor-liquid curve and solid-liquid curve in the phase diagram using three potential models which can help us to realize the feature of different phase.