TRAVIS (“Trajectory Analyzer and Visualizer”) is a program package for post-processing and analyzing trajectories from molecular dynamics and Monte Carlo simulations, mostly focused on molecular condensed phase systems. It is an open source free software licensed under the GNU GPL, is platform independent, and does not require any external libraries. Nine years after the original publication of TRAVIS, we highlight some of the recent new functions and features in this article. At the same time, we shortly present some of the underlying algorithms in TRAVIS, which contribute to make trajectory analysis more efficient. Some modern visualization techniques such as Sankey diagrams are also demonstrated. Many analysis functions are implemented, covering structural analyses, dynamical analyses, and functions for predicting vibrational spectra from molecular dynamics simulations. While some of the analyses are known since several decades, others are very recent. For example, TRAVIS has been used to compute the first ab initio predictions in the literature of bulk phase vibrational circular dichroism spectra, bulk phase Raman optical activity spectra, and bulk phase resonance Raman spectra within the last few years.

We review several methods for the calculation of vibrational spectra from ab initio molecular dynamics (AIMD) simulations and we present a new implementation in the trajectory analyzer TRAVIS. In particular, we show mass-weighted power spectra, infrared spectra, and Raman spectra with corresponding depolarization ratios, which are based on time-correlation functions of velocities, dipole moments, and polarizabilities, respectively. Using the four organic molecules methanol, acetone, nitromethane, and pinacol as test systems, we compare the spectra from AIMD simulations of the isolated molecules in gas phase to static calculations relying on the harmonic approximation and to experimental spectra recorded in a nonpolar solvent. The AIMD approach turns out to give superior results when anharmonicity effects are of particular importance. Using the example of methanol, we demonstrate the application to bulk phase systems, which are not directly accessible by static calculations, but for which the AIMD spectra also provide a very good approximation to experimental data. Finally, we investigate the influence of simulation time and temperature in the AIMD on the resulting spectra.

Potential energy curves for the dissociation of cation–anion associates representing the building units of ionic liquids have been computed with dispersion corrected DFT methods. Non-local van der Waals density functionals (DFT-NL) for the first time as well as an atom pair-wise correction method (DFT-D3) have been tested. Reference data have been computed at the extrapolated MP2/CBS and estimated CCSD(T)/CBS levels of theory. The investigated systems are combined from two cations (1-butyl-3-methylimidazolium and tributyl(methyl)posphonium) and three anions (chloride, dicyanamide, acetate). We find substantial stabilization from London dispersion energy near equilibrium of 5–7 kcal mol⁻¹ (about 5–6% of the interaction energy). Equilibrium distances are shortened by 0.03–0.09 Å and fundamental (inter-fragment) vibrational frequencies (which are in the range 140–180 cm⁻¹) are increased by typically 10 cm⁻¹ when dispersion corrections are made. The dispersion-corrected hybrid functional potentials are in general in excellent agreement with the corresponding CCSD(T) reference data (typical deviations of about 1–2%). The DFT-D3 method performs unexpectedly well presumably because of cancellation of errors between the dispersion coefficients of the cations and anions. Due to self-interaction error, semi-local density functionals exhibit severe SCF convergence problems, and provide artificial charge-transfer and inaccurate interaction energies for larger inter-fragment distances. Although these problems may be alleviated in condensed phase simulations by effective Coulomb screening, only dispersion-corrected hybrid functionals with larger amounts of Fock-exchange can in general be recommended for such ionic systems.

In this paper we show by using static DFT calculations and classical molecular dynamics simulations that the charge transfer between ionic liquid ions plays a major role in the observed discrepancies between the overall mobility of the ions and the observed conductivities of the corresponding ionic liquids, while it also directly suppresses the association of oppositely charged ions, thus the ion pairing. Accordingly, in electrochemical applications of these materials it is important to consider this reduction of the total charges on the ions, which can greatly affect the performance of the given process or device in which the ionic liquid is used. By slightly shifting from the salt-like to a molecular liquid-like system via the decreased charges, the charge transfer also fluidizes the ionic liquid. We believe that this vital information on the molecular level structure of ionic liquids offers a better understanding of these materials, and allows us to improve the a priori design of ionic liquids for any given purpose.