Effect of the density on the infrared spectra of liquid ethene by molecular dynamics simulations J.-M. Simon #,§ , A. Decrette # , K.S. Smirnov*, J.-P. Bellat # , S. Marcati # # Laboratoire de Recherches sur la R´ eactivit´ e des Solides, UMR 5613 du CNRS, Universit´ e de Bourgogne, BP 47870, 21078 Dijon Cedex, France *Laboratoire de Spectrochimie Infrarouge et Raman, UMR 8516 du CNRS, Bˆat. C5, Universit´ e des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq Cedex, France § Author to whom correspondence should be addressed. E-mail: jmsimon@u-bourgogne.fr Abstract A study of the influence of density on the thermodynamic characteristics and dynamical behavior of ethene molecules in liquid phase at 123 K is performed by molecular dynamic simu- lation technique. The simulations were carried out using an all atom model of ethene molecule and for the density of the system in the range from 600 to 900 kg.m -3 and at 1.26 kg.m -3 in the gas phase. The calculated change of the Gibbs free energy with pressure does not reveal any phase transition of the system under the external conditions used in the calculations. The analysis of the infrared spectra shows strong modifications upon the increasing of the density, in agreement with experimental results. Firstly, going from gas to liquid phase, the change in rotational motion of the molecules modifies the shape of the infrared bands and secondly, the frequencies of the bands shift upwards while the pressure increases. The data obtained are discussed in relation to studies of molecules adsorbed in microporous materials. 1