Optimization of Lennard-Jones potential parameters and benchmark comparison between ion mobility calculators in free molecular regime

Abstract

Ion Mobility Spectrometry (IMS) is a widely used technique to differentiate charged particles in the gas phase. Although there has been a significant computational development over the past few decades for calculating Ion Mobility and Collision Cross Section (CCS), still there is a need to develop it in terms of its efficiency and performance, to better understand the dynamics of the collision. The work presented here demonstrates the efficiency and performance of newly developed mobility calculator: IMoS. The results were compared to MOBCAL and were found to be in a good agreement for He and N2 for the same input parameter. IMoS, which has an ability to be parallelized, gave similar values for CCS (within 1% of error) with a speed of two order of magnitude, which is higher than that of MOBCAL. Various options of approximations such as Diffused Trajectory Methods (DHSS, TDHSS) with and without partial charges and Projected Area approximation were considered in this work which lead us to reduce the total computational time required for the calculations. A careful computational study was carried out for 47 organic molecules and few large biomolecules (> 10000 atoms) to demonstrate the similarity and differences in two widely used mobility calculator – IMoS and MOBCAL. As the calculations were made faster using IMoS, it was a necessary step to develop an optimization algorithm in order to optimize the Lennard-Jones potential parameters for gas phase calculations used in the Trajectory Method. The process of optimization follows a multiple iterative path, wherein the parameters are completely optimized for all the given elements. A surface plot was generated using tens and thousands of data points for C, H, N, O, and F to study the relationship between epsilon (ε) and sigma (σ) for each element in the N2 buffer gas. The function (F) used here is a function of experimental CCS and IMoS generated CCS, which was minimized in the process of optimization. These optimized values can be used in the mobility calculator for calculating accurate Collision Cross Sectional values.