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We extend Expanded Wang-Landau (EWL) simulations beyond classical systems anddevelop the EWL method for systems modeled with a tight-binding Hamiltonian. Wethen apply the method to determine the partition function and thus allthermodynamic properties, including the Gibbs free energy and entropy, of thefluid phases of Si. We compare the results from quantum many-body (QMB) tightbinding models, which explicitly calculate the overlap between the atomicorbitals of neighboring atoms, to those obtained with classical many-body forcefields (CMB), which allow to recover the tetrahedral organization in condensedphases of Si through e.g. a repulsive 3-body term that favors the idealtetrahedral angle. Along the vapor-liquid coexistence, between 3000K and 6000K,the densities for the two coexisting phases are found to vary significantly (by$5$ orders of magnitude for the vapor and by up to 25% for the liquid) and toprovide a stringent test of the models. Transitions from vapor to liquid arepredicted to occur for chemical potentials that are $10-15$% higher for CMBmodels than for QMB models, and a ranking of the force fields is provided bycomparing the predictions for the vapor pressure to the experimental data. QMBmodels also reveal the formation of a gap in the electronic density of statesof the coexisting liquid at high temperatures. Subjecting Si to a nanoscopicconfinement has a dramatic effect on the phase diagram, with e.g. at 6000K adecrease in liquid densities by about 50% for both CMB and QMB models and anincrease in vapor densities between 90% (CMB) and 170% (QMB). The resultspresented here provide a full picture of the impact of the strategy (CMB orQMB) chosen to model many-body effects on the thermodynamic properties of thefluid phases of Si.

journal of chemical physics online/the journal of chemical physics/journal of chemical physics

Evaluation of the grand-canonical partition function using Expanded  Wang-Landau simulations. IV. Performance of many-body force fields and  tight-binding schemes for the fluid phases of Silicon