Thermodynamic concepts in adaptive resolution simulations

This thesis work is devoted to the conceptual and technical development of the Adaptive Resolution Scheme (AdResS), a molecular dynamics method that allows the simulation of a system with different levels of resolution simultaneously. The simulation domain is divided into high and low resolution zones and a transition region that links them, through which molecules can freely diffuse.rnThe first issue of this work regards the thermodynamic consistency of the method, which is tested and verified in a model liquid of tetrahedral molecules. The results allow the introduction of the concept of the Thermodynamic Force, an external field able to correct spurious density fluctuations present in the transition region in usual AdResS simulations.rnThe AdResS is also applied to a system where two different representations with the same degree of resolution are confronted. This simple test extends the method from an Adaptive Resolution Scheme to an Adaptive Representation Scheme, providing a way of coupling different force fields based on thermodynamic consistency arguments. The Thermodynamic Force is successfully applied to the example described in this work as well.rnAn alternative approach of deducing the Thermodynamic Force from pressure consistency considerations allows the interpretation of AdResS as a first step towards a molecular dynamics simulation in the Grand Canonical ensemble. Additionally, such a definition leads to a practical way of determining the Thermodynamic Force, tested in the well studied tetrahedral liquid. The effects of AdResS and this correction on the atomistic domain are analyzed by inspecting the local distribution of velocities, radial distribution functions, pressure and particle number fluctuation. Their comparison with analogous results coming from purely atomistic simulations shows good agreement, which is greatly improved under the effect of the external field.rnA further step in the development of AdResS, necessary for several applications in biophysics and material science, consists of its application to multicomponent systems. To this aim, the high-resolution representation of a model binary mixture is confronted with its coarse-grained representation systematically parametrized. The Thermodynamic Force, whose development requires a more delicate treatment, also gives satisfactory results.rnFinally, AdResS is tested in systems including two-body bonded forces, through the simulation of a model polymer allowed to adaptively change its representation. It is shown that the distribution functions that characterize the polymer structure are in practice not affected by the change of resolution.rnThe technical details of the implementation of AdResS in the ESPResSo package conclude this thesis work.