Tom FRÖMBGEN1,2, Jan BLASIUS1, Vahideh ALIZADEH1, Barbara KIRCHNER1
1Mulliken Center for Theoretical Chemistry, University of Bonn, Bonn, Germany
2International Max Planck Research School on Reactive Structure Analysis for Chemical Reactions (IMPRS-RECHARGE), Mülheim, Germany
When considering a liquid system, the bulk phase is often assumed to behave completely homogeneously, i.e., all molecules of the bulk behave identically and contribute equally to the chemical and physical properties of the system. However, there is a plethora of indications in the literature that clusters of finite and distinct size play a significant role in liquid systems or even dominate them. Aiming to understand the physical and chemical properties of ionic liquids (ILs), it is of utmost importance to analyze their structures on a molecular level. The formation of clusters is known to occur in ILs and to influence structural and dynamical quantities, such as radial pair distributions, transport properties, and vibrational spectra.
To the present date, different tools to analyze clustering are already available, most of which are developed for a certain target system or application, e.g. interfaces, surfaces, proteins etc. Hence, these tools often lack a general applicability to arbitrary systems. In accordance, we found an ongoing need for tools to investigate and quantify the existence of clusters in various chemical systems in order to describe the liquid phase accurately. To address this issue, we developed a novel cluster analysis and implemented it into the open source software TRAVIS. The underlying algorithm is exclusively based on atom distances and does not require a user-directed (and biased) geometric cutoff criteria. TRAVIS is a program package to analyze molecular dynamics and Monte-Carlo simulations, and well-known in the IL community. In this contribution, we will briefly present our recently published cluster analysis and focus on its application to IL and IL/water systems which reveals interesting changes of the clustering behavior of different IL species in the presence of water.