An open-source implementation of a quasi harmonic approach to compute the intramolecular entropy of particle systems
The implementation of a quasi harmonic approach to determine the intramolecular entropy of particle systems in an open-source python code is presented. The code makes use of platform-independent tools and can be readily transferred to any computer platform which can run python. It is straightforward to incorporate the code into the LAMMPS software as well as into other molecular simulation programs. In the computational part of this work, we evaluate the intramolecular entropy of atactic polystyrene and polyethylene melts in an all atom as well as in a coarse grained (CG) resolution at five temperatures between 400 and 450 K. For both polymers, coarse graining with its reduced system degrees of freedom and its softened bonded radial potential, causes a strong reduction of the entropy. However, the averaged entropy per degree of freedom in the CG case exceeds the one under atomistic resolution. Also the temperature dependence of the intramolecular entropy becomes weaker in the transition from an atomistic to a coarse grained description. The larger entropy of polyethylene in comparison to polystyrene under both resolutions is a consequence of its longer chain length. To simplify a comparison of the two polymers in the two considered resolutions, we analyze the total intramolecular entropy as well as its temperature gradient via a scaling of both quantities by the number of degrees of freedom. In the two studied systems and for both levels of resolution, a linear dependence of the intramolecular entropy on the temperature is identified.