In this thesis collision-induced absorption (CIA) coefficients are computed using molec-ular dynamics (MD) simulations. Part I is dedicated to the theoretical frame of the method, from the classical theory radiation to the derivation of an absorption coefficient. The second part is a on the implementation of the method in the in-house software Spa-CIAL (Spectra of Collision-Induced Absorption with LAMMPS). This package is split in two parts: the molecular dynamics part being treated with the open source package LAMMPS, and the post-processing for the computation of the collision-induced absorp-tion with a Python code. The post-processing has been developed in two distinct ways each of them presenting different properties. The first one, based on what has been done previously, is designed to compute the dipole auto-correlation function (ACF) to obtain the CIA spectra after Fourier transformation. Many improvements has been made like the time averaging method is used in order to considerably increase the statistics requiring reasonable resource needs. The use of the fast Fourier transform algorithm (FFT) and the apodization procedure are also used for better accuracy of the results. The reformulation of the equations, especially with the Wiener-Kintchine (WK) theorem, gives a completely new implementation for which the CPU intensive computation of the dipole ACF is no longer needed. Instead, the contributions to the CIA spectrum are computed for each pair separately. In addition to improve significantly the performance of the code, it is now possible to separate the free-free and the bound-bound contributions. The comparison with the previous method (ACF) for the Ar-Xe system has shown a good accordance thus validating this new implementation. This great progress paves the way for the classical study of the dimers features in the absorption coefficient. The programs developed in this work can be adapted to handle molecular gas mixtures that are relevant in studies of radiative transfer in planetary atmospheres.

With the reformulation of the classical equations of collision-induced absorption, we present a method to perform the direct computation of the spectral density function. This way the absorption coefficient can be computed from classical molecular dynamics (MD) without the computationally demanding evaluation of the dipole autocorrelation function. In addition, we have developed an algorithm to extract the bound-to-bound dimer contribution to the MD simulated absorption. The method has been tested on the Ar–Xe rare gas system. Comparisons with quantum mechanical (QM) and conventional MD methods validate the approach. The obtained MD bound-to-bound spectra generally agree in shape and magnitude with QM results, including features stemming from rotations and vibrations of the Ar–Xe dimer.

We continue the development of the in-house molecular dynamics software package SpaCIAL and test it for the computation of the collision-induced absorption coefficients for a neon (Ne) and krypton (Kr) gas mixture. An apodization procedure for the dipole autocorrelation function is implemented and tested. We also carry out a statistical study of the convergence rate with respect to ensemble size. The resulting absorption coefficients show a good accordance with quantum mechanical results. Comparison with laboratory measurements shows agreement within 10%–20% at T = 295 K. At T = 480 K, a larger difference of 40%–80% is observed, which can presumably be explained by experimental uncertainties. For the study, an empirical (Barker, Fisher, and Watts) interaction-potential [Mol. Phys. 21, 657 (1971)] for Ne–Kr has been developed. Ab initio {coupled cluster with singles and doubles (triples) [CCSD(T)]} potentials for Ne–Ne, Kr–Kr, and Ne–Kr have been computed, as well as the CCSD(T) interaction-induced Ne–Kr dipole moment curve.

We have implemented a scheme for classical molecular dynamics simulations of collision-induced absorption. The program has been applied to a gas mixture of argon (Ar) and krypton (Kr). The simulations are compared with accurate quantum dynamical calculations. The comparisons of the absorption coefficients show that classical molecular dynamics is correct within 10% for photon wave numbers up to 220 cm−1 at a temperature of 200 K for this system. At higher temperatures, the agreement is even better. Molecular dynamics accounts for many-body interactions, which, for example, give rise to continuous dimer formation and destruction in the gas. In this way, the method has an advantage compared with bimolecular classical (trajectory) treatments. The calculations are carried out with a new empirical Ar–Kr pair potential. This has been obtained through extensive analysis of experimental thermophysical and transport properties. We also present a new high level ab initio Ar–Kr potential curve for comparison, as well as ab initio interaction-induced dipole curves computed with different methods. In addition, the Ar–Kr polarizability and hyperpolarizability are reported. A comparison of the computed absorption spectra with an experiment taken at 300 K shows satisfactory agreement although a difference in absolute magnitude of 10%–15% persists. This discrepancy we attribute mainly to experimental uncertainty.

We have developed an empirical Barker, Fisher and Watts (BFW) interatomic potential for the Ar–Kr pair along with a dipole moment computed from first principles using Møller–Plesset perturbation theory to second order (MP2). Using these results, we performed molecular dynamics calculations to compute the Ar–Kr collision induced absorption (CIA) spectrum at different temperatures. By comparing them to other calculations using a two body interaction treated with quantum mechanics, we have shown that the difference is due to the dimer's contribution which grows in importance as the temperature is lowered.

We pursue simulations of collision-induced absorption in a mixture of argon and xenon gas at room temperature by means of classical molecular dynamics. The established theoretical approach (Hartmann et al. 2011 J. Chem. Phys. 134 094316) is implemented with the molecular dynamics package LAMMPS. The bound state features in the absorption spectrum are well reproduced with the molecular dynamics simulation in comparison with a laboratory measurement. The magnitude of the computed absorption, however, is underestimated in a large part of the spectrum. We suggest some aspects of the simulation that could be improved

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