plll d [ ELSEVIER Physica A 245 (1997) 393-410 Analytical treatment of the fused hard-sphere chain model. 0.5 < L < 1 Yurij Duda a, 1,,, Eduard Vakarin b, Yurij Kalyuzhnyi b, Myroslav Holovko b a Instituto de Quimica de la UNAM, Circuito Exterior, Coyoac{m 04510, Mkxico DF, Mexico b Institute for Condensed Matter Physics, National Academy of Sciences of the Ukraine, Lviv 11, Ukraine Received 2 September 1996; revised 7 April 1997 Abstract Multidensity integral equation theory for a model of an associating fluid forming freely jointed fused hard-sphere chain, is presented. Our approach is based on the Wertheim's polymer Percus- Yevick (PPY) theory supplemented by the ideal chain approximation and can be regarded as an extension of the PPY theory for tangent hard-sphere fluids proposed by Chang and Sandler (J. Chem. Phys. 102, 1995, 437). The radial distribution function and the structure factor are calculated for the different model parameters. We compare the resulting predictions for the in- termolecular distribution function with the Monte Carlo simulation results for the fused diatomic system. It is found that the accuracy of the prediction of the structure of such system is reali- able in a rather wide range of density. It is shown that structure factor exhibits a peculiarity (so-called pre-peak) at small wave numbers, connected with the formation of relatively large molecular aggregates. The dependence of the pre-peak magnitude on the degree of penetrability is investigated and discussed. PACS: 61.25.Hq; 61.20.Gy; 61.25.Em; 61.41.+e Keywords: Chains; Integral equations; Radial distribution function; Structure factor I. Introduction The investigation of the equilibrium properties of macromolecular systems, in par- ticular, systems which consist of chain molecules, has recently received a great deal * Corresponding author. Yurij Duda Tel.: 525 622 44 39; fax: 525 616 22 03; e-mail: yduda@servidor. unam.lnx. I Permanentaddress: Institute for CondensedMatter Physics, National Academy of Sciences of the Ukraine, Lviv 11, Ukraine. 0378-4371/97/$17.00 Copyright (~) 1997 Elsevier Science B.V. All rights reserved PH S0378-4371 (97)00308-7