Ab-Initio Simulation of Solid State Polymerization of Acetylene Under Pressure
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2,
3 3 M. Parrinello 2 , G.L. 3 4 Chiarotti , P. Focher and
E. Tosatti '
'Istituto Nazionale Fisica della Materia and Dipartimento di Scienza dei Materiali, 2 3
UniversitA di Milano, Via Emanueli 15, 1-20126 Mialno, Italy Max-Planck Institut fiir Festkbrperforschung, Heisenbergstr. 1, D-70569 Stuttgart,
Germany Istituto Nazionale Fisica della Materia and International School for Advanced Studies, Via Beirut 4, 1-34014 Trieste, Italy "4International Centre for Theoretical Physics, P.O. Box 586, 1-34014 Trieste, Italy
ABSTRACT We have recently devised a new method for the ab initio simulation of solid-solid phase transformations under pressure. The method is here applied to the study of the solid-state polymerization of acetylene under pressure and the generation of hydrogenated amorphous carbon from compression of polyacetylene.
1
Introduction
Existing ab-initio theory of solid-solid phase transitions under pressure has been mostly restricted to the study of relative enthalpies of known structural phases at different volumes and at zero temperature. We have recently developed a constant pressure ab-initio molecular dynamics (MD) method [1, 2] which allows a dynamical simulation of structural phase transitions under real conditions of temperature and pressure by first-principles. The method efficiently combines the Car-Parrinello method [3] for ab-initio MD with the Parrinello-Rahman variable shape method [4]. Unlike standard fixed cell MD, within this scheme phase transformations may spontaneously take place during the simulation with variation of external pressure and/or temperature. The prediction of new phases, still retaining first-principles accuracy, may thus be obtained without any initial guess on the final stable structure. The method has been applied so far to the study of several solids at high pressure Si [1], C [5], ice [6], CH 4 [7] and H2 [8]. In the following we report on recent applications of the method to the study of the solid-state polymerization of acetylene under pressure [9] and the generation of anisotropic hydrogenated amorphous carbon by compression of polyacetylene [10].
407 Mat. Roe.Soc. Symp. Proc. Vol. 499 01998 Materials Research Society
2
Method
Following the idea of Parrinello and Rahman (PR) [4] for the classical MD, we have extended the fixed-cell original Car-Parrinello (CP) [3] scheme, by including the possibility of shape and volume fluctuations of the simulation cell. The basic idea is to consider an extended lagrangian, where the edges of the simulation cell a, b, c are additional degrees of freedom, whose trajectories are determined by appropriate generalized forces. Defining the matrix h = (a, b, c), the position R of a particle in the cell can be written in terms of the scaled coordinates S as: R= hS. The presence of the electronic wave-functions in the CP formulation makes the definition of this fictitious system slightly subtler here than in the classical case. The Kohn-Sham wave-functions 'Oh(r), defined and normalized on cell h, are not independent fields
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