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MM: Fachverband Metall- und Materialphysik

MM 12: Postersitzung I

MM 12.22: Poster

Monday, March 14, 2011, 17:30–19:00, P5

Extending the Cluster Expansion by vibrational free energies — •David Reith, Markus Stöhr, and Raimund Podloucky — Faculty of Chemistry, Univ. Vienna

The Cluster Expansion technique allows to carry over the precision of density functional theory (DFT) calculations to systems consisting of 10⁶ - 10⁹ atoms [1]. It is based on DFT total energies of properly chosen input structures with a common basic lattice. The effective cluster interaction energies (ECI) are derived from CE fitting criteria and then enter into Monte Carlo (MC) simulations for e.g. deriving phase boundaries. By adding vibrational free energies to DFT total energies the ECIs become temperature dependent which strongly influences the CE+MC treatment. The temperature dependent extension of CE is demonstrated on the binary Fe_1−xC_x alloy system, for which it turns out that including the vibrational free entropy strongly increases the solubility of Cu in Fe-rich (steel) alloys. We discuss the complications and strategies which have to be followed for deriving the ECIs and for combining CE with temperature dependent ECIs and MC. For the CE+MC calculations we used the UNCLE code [1] and a direct force constant approach for deriving the phonon spectra [2] for each of the DFT input structures.

[1] D. Lerch, O. Wieckhorst, G.L.W. Hart, R.W. Forcade, and S. Müller, Modelling Simul. Mater. Sci. Eng. 17, 055003 (2009).

[2] D. Reith, program package fPHON based on PHON by D. Alfè, 1998.