等溫布朗型分子動力模擬對銀叢集手征對稱之探討

Chirality in Ag clusters by the isothermal Brownian-type molecular dynamics simulations

曾慶哲(Ching-Che Tseng)

指導教授 ： 吳天鳴 ； 賴山強

國立交通大學/理學院/物理研究所/碩士(2020年)

摘要

DFTB理論常見用於配合MBH理論來計算能量函數且會被用來計算及預測金屬叢集，就像本論文中有提到的銀叢集及金叢集，銀叢集分別從三顆原子計算至二十顆原子，也一樣是採用DFTB及MBH理論來做預測，在三顆到二十顆原子且搭配理論的預測下我們得知了在Ag14、Ag17及Ag20之最低能量結構有著手征對稱現象的出現，Ag14、Ag17及Ag20皆是有著相同能量但彼此結構為互相對稱的現象發生，普遍我們所預期的情況為有著相同能量必定有著相同之結構，Ag14、Ag17及Ag20打破了我們舊有的認知，所以我們藉由等溫布朗型分子動力模擬去了解Ag14、Ag17及Ag20從左邊結構如何演變到右邊結構的過程，且我們嘗試要去微觀地瞭解說原子會如何運動及其中左右互相演變的機制，從裡面找到一些規律及規則，為了要能夠理解左右結構轉變的機制，我們首先要在模擬的過程中找到左右對稱之結構，在左右互相演變的過程中勢必會出現左右對之結構，我們要藉由左右對稱結構來做接下來的分析如:reaction coordinate，在定義好reaction coordinate後，我們就可以接著做histogram的統計，我們也可以藉著以上的分析方式來分析其他也有著手征對稱之叢集。

關鍵字

銀叢集 ； 手征對稱 ； DFTB理論 ； MBH理論

並列摘要

Modified parameters in density functional tight-binding (DFTB) are proposed [Oliveira et al., J. Phys. Chem. A 120 (2016) 8469-8483] and used to calculate the energy function which is then combined with an elegant modified basin hopping (MBH) optimization algorithm to systematically study the coinage metal clusters Agn, n=3,…,20. It was predicted by the DFTB/MBH optimization method that, among the 18 clusters, the lowest energy geometries of Ag14, Ag17 and Ag20 clusters exhibit chiral characteristics, i.e. these clusters each has a same energy value but possesses two enantiomers showing mirror-image symmetry. This unexpected discovery of left-right enantiomers at the DFT-based DFTB level will be investigated by the isothermal Brownian-type molecular dynamics simulation to understand how an enantiomer, say left, transits to its right enantiomer, the mechanism of these enantiomeric transitions in terms of the dynamics of atoms, the transition events,.., etc. To unravel deeper the transition dynamics, efforts were made to search for the intermediate symmetrical structure from the simulations data and deduce from the latter a reaction coordinate that hopefully can provide more information on such interesting issue as transition rate between enantiomers, the frequency of enantiomeric transitions, the mechanism of transition in terms of atoms involved,.., etc. All three chiral clusters will be studied and compared to show their similarities and differences

並列關鍵字

chiral clusters ； enantiomeric transition ； molecular dynamics ； DFTB theory

參考文獻

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