8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Fri 911:10-14:00P4.61Free energy and pressure calculations withintwo-dimensional Wang-Landau entropic samplingNikolay Volkov, 1 Pavel Vorontsov-Velyaminov, 2 and Alexander Lyubartsev 31 Department of Chemistry, University of Gothenburg, Kemigården 4, SE-412 96,Göteborg, Sweden2 St. Petersburg State University, Faculty of Physics, St. Petersburg, Russian Federation3 Division of Physical Chemistry, Stockholm University, Stockholm, SwedenIn Monte Carlo simulations the pressure between non-uniformly charged surfaces in a presence ofelectrically charged species can not be evaluated by the use of contact value theorem. One wayto calculate it is to estimate kinetic (ideal), collisional and electrostatic contributions by studyingatomic configurations. Alternatively one can evaluate pressure as a derivative of free energy withrespect to separation between the surfaces along Z-axis provided that periodic boundary conditionsare imposed in X and Y directions. Using approach based on entropic sampling (ES) within theWang-Landau (WL) algorithm we calculate Helmholtz free energy and pressure for electrolytesbetween two planar surfaces, as exemplified by simulation of a polyelectrolyte between membranesurfaces. Membranes are presented by parallel plane surfaces holding either fixed or mobiledipoles (representing lipid headgroups). A strongly charged polyion accompanied by neutralizingcounterions is placed between the membranes. Within two-dimensional ES-WL approach we obtainjoint density of states as a function of energy and volume in a single run. Then Helmholtz freeenergy over wide temperature and volume ranges is calculated from density of states by simpleintegration. The osmotic pressure is evaluated as a derivative of free energy. Average volume asa function of pressure is determined under condition of NPT ensemble which gives an alternativeway to produce P(V) isotherms.61