Many-body correlations in multipole-induced dipole absorption spectra of liquid OCS

Time correlation functions for several multipole-induced dipole moments corresponding to absorption in the far-infrared spectral region have been evaluated from molecular dynamics computer simulations of liquid carbonyl sulfide (OCS). Five state points along the liquid-vapor coexistence curve have been investigated. The dipole-, quadrupole- and octopole-induced dipole moments have been estimated for the interacting pairs of molecules in the liquid. The induced moments are produced by the action of the electric fields of the molecular electrostatic multipoles upon the anisotropic polarizabilities of the OCS. The component n-body (n = 2,3,4) time correlation functions for these multipole-induced dipole moments as well as those for the cross-correlation functions between the induced and the permanent dipole moments of these molecules have been calculated. It is shown that the n-body terms involved in the time correlation functions containing quadrupole-induced dipole moments cancel each other at long times. Furthermore, 4-body correlations are significantly less important than the 2- and 3-body contributions for all induction mechanisms investigated here. (C) 1997 Elsevier Science B.V.