An empirical fundamental equation of state correlation in terms of the Helmholtz energy is presented for hexamethyldisiloxane. The relatively small amount of thermodynamic data that is available in the literature for this substances is considerably extended by speed of sound measurements and numerical results for Helmholtz energy derivatives from molecular modeling and simulation. The speed of sound apparatus employed in this work is based on the pulse-echo technique and operates up to 150 MPa in the temperature range between 250 K and 600 K. The range of validity of the equation of state, based on laboratory data from literature and speed of sound data of this work, is from 270 K to 580 K and up to 130 MPa. Molecular simulation data are applied to extend the range of validity up to 1200 K and 600 MPa. (C) 2015 Elsevier B.V. All rights reserved.