Ion Mobility Suite (IMoS) 2: A General Computational Framework for Solving the Generalized Boltzmann Equation with Rotational Transport for Polyatomic Ions in Arbitrary Fields

Ion mobility spectrometry (IMS) depends on accurate descriptions of ion–neutral collision dynamics to relate measured mobilities to molecular structure; however, existing approaches based on collision integral quadratures or rigid-body trajectory methods are largely restricted to low-field, elastic, and near-equilibrium assumptions. Here we present IMoS 2, a trajectory-resolved, first-principles computational framework that directly realizes the generalized Boltzmann equation with Curtiss-type rotational transport and a Wang–Chang–Uhlenbeck (WCU) collision operator for polyatomic ions in arbitrary electric fields. Rather than evaluating high-dimensional collision integrals, the method propagates ions within an explicit neutral gas environment using Monte Carlo molecular dynamics, explicitly coupling translational and rotational motion and permitting multiple simultaneous collisions. This strategy provides, in a single simulation, time-resolved trajectories, translational and angular velocity distribution functions, and transport observables such as drift velocity and mobility. In the low-field limit, IMoS 2 quantitatively reproduces Mason–Schamp mobilities and agrees with IMoS 1 and conventional quadrature methods in He and N2. Temperature-dependent trends match available experiments, and simulations under arbitrary field profiles agree with two-temperature and collision-integral predictions for light monatomic gases, while deviations at higher fields in heavier gases are captured and attributed to inelastic energy transfer into internal degrees of freedom. To our knowledge, IMoS 2 provides the first direct numerical realization of the Curtiss–Wang–Chang–Uhlenbeck kinetic equation for polyatomic ions, establishing a general platform that recovers classical transport theory where valid and extends IMS modeling to nonequilibrium, field-dependent regimes beyond traditional quadrature approaches.