The fast dynamics of benzene in the liquid phase Part II.¤ A molecular dynamics simulation R. Chelli,ab G. Cardini,ab M. Ricci,ace P. Bartolini,ad R. Righini*abd and S. Califanoab a L ENS, University of Florence, 50125 Florence, Italy. E-mail : righini=chim.uniÐ.it b Department of Chemistry, University of Florence, 50121 Florence, Italy c Department of Chemistry, University of Basilicata, 85100 Potenza, Italy INFM Firenze d Unita ` INFM Napoli e Unita ` Received 26th January 2001, Accepted 20th April 2001 First published as an Advance Article on the web 25th June 2001 A molecular dynamics simulation is performed for liquid benzene in the rigid body approximation. The results concerning the structural and dynamical properties of the system provide the basis for the interpretation of recent experimental data. In particular, it is shown that the system is characterised by a well deÐned cage structure, and that the average dynamics of the cages describe the main dynamical features of the bulk liquid. The calculated mean lifetime of the cages is in good agreement with the value of the Kubo correlation time derived from the experiments. In the picture emerging both from experiments and calculations, the fast intermolecular dynamics of liquid benzene is characterised by an inhomogeneously broadened distribution of intermolecular vibrational frequencies, whose dephasing is primarily due to the relaxation of the local structures. In particular, this mechanism is responsible for the dephasing of the low frequency librations giving rise to the intermediate quasi-exponential relaxation observed in the optical Kerr e†ect experiments. I Introduction In the previous paper1 (hereafter referred as paper I) we reported on the time resolved optical Kerr e†ect (OKE) inves- tigation of the fast dynamics of benzene in the liquid phase. Besides the picosecond exponential decay due to rotational di†usion of the molecules (tumbling), at short times (\5 ps) the experiments show evidence of intermolecular oscillatory dynamics, and of some intermediate regime. In the interpreta- tion proposed in paper I, in accord with the conclusions of ref. 2, we relate the oscillatory and the intermediate regimes to the inhomogeneous distribution of the intermolecular librational frequencies, and attribute the peculiar temperature-dependent shape of the intermolecular spectrum to motional narrowing e†ects. In this paper the dynamics of liquid benzene is investi- gated at a microscopic level by means of molecular dynamics (MD) simulations. MD in fact can provide basic information, not achievable with other techniques, on the structure of the liquid and on its dynamical properties at the molecular scale. Several computer simulations of liquid benzene can be found, also in recent years,3 h5 in the literature. In particular, some of us have published recently5 the results of a MD simulation of liquid benzene. The main goal of that investigation was under- standing the relaxation mechanisms of the intramolecular vibrations ; however, some of the results reported there are of interest also for the description of the structure and of the rotational and translational dynamics of the liquid. Here we extend and complete that simulation, taking into account dif- ferent models for the intermolecular potential, and considering the e†ect of changing the sample temperature ; we also calcu- late the low frequency Raman spectral density, and compare it to the experimental optical spectra reported in paper I. ¤ For Part I see ref. 1. According to the results of the experimental investigation described in paper I, the most important answers we expect from the MD simulations concern the following points : (i) The translational and orientational ordering (if any) of the liquid, obviously related to the role played by the cage dynamics, as discussed in paper I ; (ii) the distribution of inter- molecular vibrational frequencies and, in particular, of the components corresponding to librations about in-plane axes (tumbling librations), that are expected to give the largest con- tribution to the optical activity ; (iii) the characteristic param- eters deÐning the cage structure and dynamics, such as its mean lifetime, to be related to the correlation time of the q c stochastic perturbations that appears in the Kubo treatment of the spectral line shape (see paper I) ; (iv) the ability of the computational method to reproduce the optical spectra obtained by Fourier transforming the time domain experi- mental data. In the next section the model potential and the computa- tional details are described. In the third section the calculated structural and dynamical properties of liquid benzene are reported and discussed and the calculated low frequency vibrational Raman spectral density is compared to the experi- mental results of paper I. Finally, the last section summarises some general conclusions based on the results of the present paper and on those of paper I. II The calculation The intermolecular potential It is generally accepted that the intermolecular and intramole- cular vibrations of benzene in the condensed phases are dynamically separated : the lowest intramolecular vibration in DOI : 10.1039/b100943p Phys. Chem. Chem. Phys., 2001, 3, 2803È2810 2803 This journal is The Owner Societies 2001 (