基於量子化學方法研究惰性氣體-甲烷分子間位勢能及甲烷流體之分子動力學模擬

Abstract

我們使用HF, MP2與DFT方法做 “惰性氣體-惰性氣體” 、 “惰性氣體-甲烷分子”的原子與分子間作用力計算，並分析其計算上的差異，也使用多組不同的基底計算，HF的計算主要產生排斥作用力，DFT的計算結果將與較精確的MP2計算結果將互比較，找出較適合惰性氣體-甲烷量子化學計算的函數配對。最後我們將量子化學計算結果帶入混合理論擴大驗證惰性氣體與甲烷分子是否有吻合我們所預測理論值。在混合理論的驗證中，如果我們把甲烷分子看成一個放大的惰性氣體原子並將12個構型能量與位置加以平均，則混合理論可得到較精確的結果。 我們使用四種曲線擬合模型(4-Site，4-Site(scaled)，5-Site’，5-Site)以Lennard-Jones potentials配合12個構型甲烷-甲烷分子間作用力建構力場，並將其結果帶入牛頓方程式做分子動力學運算，模擬甲烷在T=92K， P=5MPa，T=150K, P=100MPa，T=370K， P=51MPa， 61MPa， 77MPa狀態下的平衡性質。動態性質方面我們沿著氣化曲線作自相關函數的模擬，進而求得擴散係數、剪力黏滯係數與熱傳導係數針對其值及趨勢與實驗值加以比較，比對後我們發現我們的四個曲線擬合模型在分子動力學模擬已經有相當的準確度，這可說明使用ab initio力場進行分子動力學模擬可得到良好的精確性。

We have calculated the interaction potentials of the “ Inert gas-Inert gas ” and 3 “ Inert gas-Methane ” using the Hartree-Fock (HF) self-consistent theory, the correlation-corrected second-order Møller-Plesset (MP2) perturbation theory, and the density functional theory (DFT). The HF calculations yield repulsive potentials, and the DFT calculations can compare with the accurate MP2 results to find out the suitable functions for “ Inert gas-Methane ”.Then we use Mixing rule to calculation the “Inert gas-Methane” interaction from the “Methane-Methane”and “Inert gas-Inert gas” ab initio data, and found that the result have good agreement with the “Inert gas-Methane” ab initio data ,if we regard methane molecular as a large inert gas atom. We use four kinds of site-site-fitting models to fit the results of quantum chemistry calculation.We can get the parameters for Lennard-Jones potentials of bond lengths and binding energies and then input them to build force fields. To simulate the equilibrium properties at T=92K, P=5MPa, T=150K, P=100MPa, T=370K, P=51MPa, P=61MPa, P=77MPa and dynamics properties of methane liquid along the vapor-liquid curve. 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.