We present an Instantaneous Normal Modes analysis of liquid HF aimed to clarify the origin of peculiar dynamical properties which are supposed to stem from the arrangement of molecules in linear hydrogen-bonded network. The present study shows that this approach is a unique tool for the understanding of the spectral features revealed in the analysis of both single molecule and collective quantities. For the system under investigation we demonstrate the relevance of hydrogen-bonding "stretching" and fast librational motion in the interpretation of these features.PACS numbers: 62.60.+v, 63.20.Pw In recent years the analysis of Instantaneous Normal Modes (INM) of normal and supercooled liquids has given a sound improvement to the understanding of the microscopic processes underlying the dynamical properties of these systems. Applications of the method are to be found in the calculation of macroscopic quantities through the knowledge of the density of states (e.g. the diffusion coefficient [1]) and in the interpretation of atomic motion through the inspection of the eigenvectors [2][3][4]. This last method appears to be a unique tool for the interpretation of dynamical features (single molecule or collective) of a disordered system in terms of correlated motions of its constituents. In particular it is interesting to explore how the presence of locally ordered units is reflected in the time behaviour of, for example, the velocity autocorrelation function (VACF). Attempts in this direction have to be found in the analyses of: i) the correlation function of the projection of the centre of mass (CoM) velocity of water molecules along the directions of the normal coordinates of a cluster of three molecules [5]; ii) the projection of INM eigenvectors onto the totally symmetric displacement coordinates of ZnCl 2− 4 tetrahedral units [2]. Moreover the INM approach has been exploited to describe the dynamical features of molecular liquids (e.g. diatomic Lennard-Jones [6], CS 2 [7]) including hydrogen bonded systems like water [8,3]. On the other hand recent molecular dynamics (MD) simulations of HF have revealed peculiar dynamical features, e.g. a peak at ≃ 50 ps −1 in the spectra of both collective and single molecule correlation functions [9,10]. In the collective longitudinal and transverse current spectra this mode appears to have an optical-like character, since its frequency is found to be independent of the wavevector. For the interpretation of the CoM VACF spectrum this peak has been related to the relative motion of two nearest neighbour molecules [10]. Since nearest neighbours are also hydrogen bonded it is tempting to assign this dynamical feature to hydrogen bonding "stretching". As a matter of fact HF molecules have been demonstrated to form irregular zig-zag chains of different size, being this peculiar clustering favoured by the geometry of the molecule and the strong electrostatic interaction [11]. A dynamical characterisation of these ordered units has not yet been given, so that any assignment of the sp...