期刊
MACROMOLECULAR THEORY AND SIMULATIONS
卷 20, 期 7, 页码 496-509出版社
WILEY-V C H VERLAG GMBH
DOI: 10.1002/mats.201100015
关键词
entropic sampling; membranes; Monte Carlo simulations; osmotic pressure; polyelectrolytes
资金
- Swedish Institute
- Swedish Research Council (Vetenskapsra det) [621-2007-5256]
- Russian Foundation for Fundamental research (RFFI) [02-08-0004/a]
The Monte Carlo method based on two-dimensional entropic sampling within the WangLandau (WL) algorithm is applied to simulation of a continuous model of a polyelectrolyte between membrane surfaces. Membranes are presented by parallel plane surfaces holding either fixed or mobile dipoles (representing lipid headgroups). A strongly charged polyion accompanied by neutralizing counterions is placed between the membranes. Periodic boundary conditions are imposed along X-and Y-axes. The volume of the main cell is varied during the simulation by shifting one of the surfaces along Z-axis. Within two-dimensional WL sampling algorithm we obtain joint density of states as a function of energy and volume in a single run. In order to increase efficiency of our calculations we introduce a number of modifications to the original WL-approach. Various properties of the system over wide temperature and volume or pressure ranges, i.e., conformational energy, heat capacity, and free energy, are obtained from the two-dimensional density of states by simple integration. The osmotic pressure is calculated as a derivative of Helmholtz free energy. Alternatively, properties of the system, including average volume, can be obtained under condition of NPT ensemble. It is shown that the both approaches produce coinciding P-osm(V) isotherms. In all considered cases we observe repulsive effective interaction between the membrane surfaces, and repulsion is stronger for the surfaces with fixed dipoles.
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