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In this paper we present a Path Integral Monte Carlo (PIMC) simulation of theorthorhombic phase of crystalline polyethylene, using an explicit atom forcefield with unconstrained bond lengths and angles. This work represents aquantum extension of our recent classical simulation (J. Chem. Phys. 106, 8918(1997)). It is aimed both at exploring the applicability of the PIMC method onsuch polymer crystal systems, as well as on a detailed assessment of theimportance of quantum effects on different quantities. We used the $NpT$ensemble and simulated the system at zero pressure in the temperature range 25- 300 K, using Trotter numbers between 12 and 144. In order to investigatefinite-size effects, we used chains of two different lengths, C_12 and C_24,corresponding to the total number of atoms in the super-cell being 432 and 864,respectively. We show here the results for structural parameters, like theorthorhombic lattice constants a,b,c, and also fluctuations of internalparameters of the chains, such as bond lengths and bond and torsional angles.We have also determined the internal energy and diagonal elastic constantsc_11, c_22 and c_33. We discuss the temperature dependence of the measuredquantities and compare to that obtained from the classical simulation. For somequantities, we discuss the way they are related to the torsional anglefluctuation. In case of the lattice parameters we compare our results to thoseobtained from other theoretical approaches as well as to some availableexperimental data. In order to study isotope effects, we simulated also adeuterated polyethylene crystal at a low temperature. We also suggest possibleways of extending this study and present some general considerations concerningmodeling of polymer crystals.Comment: 18 pages, RevTex, 18 figures, 3 tables, submitted to Phys. Rev.

Orthorhombic phase of crystalline polyethylene: A constant pressure path-integral Monte Carlo study