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Manuscript Title: Density-functional theory calculations for poly-atomic systems: electronic structure, static and elastic properties and ab initio molecular dynamics.
Authors: M. Bockstedte, A. Kley, J. Neugebauer, M. Scheffler
Program title: fhi96md
Catalogue identifier: ACTF_v2_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 107(1997)187
Programming language: Fortran.
Computer: IBM RS/6000.
Operating system: UNIX.
RAM: 32M words
Word size: 32
Keywords: Solid state physics, Crystal field, Density-functional, Local-density, Generalized gradient, Pseudopotentials, Plane-wave basis, Super cell, Molecular dynamics, Optimization structure, Total-energy, Potential-energy surface, Chemical binding, Diffusion surface, Reactions, Crystals, Crystals defects, Molecules.
Classification: 7.3.

Nature of problem:
In poly-atomic systems as for example molecules [1,2], crystals [3,4], defects in crystals [5,6], surfaces [7-9], it is highly desirable to perform accurate electronic structure calculations, without introducing uncontrollable approximations. This enables the predictive description of equilibrium properties as well as of non-equilibrium phenomena for a wide class of materials. Examples studied with the present code or its predecessor include meta-stabilities of defects [5-10}, surface reconstructions [11,12], diffusion [9,13], surface reactions [14-16], and phase transitions [17]. Molecular dynamics simulations combined with first-principles forces are a powerful tool to analyze the motion of the nuclei [13,18] and to accurately calculate thermodynamic properties such as diffusion constants and free energies [13]. The computer code described below enables this variey of investigations. It employs density-functional theory [20] together with the local- density approximation [21,22] or generalized gradient approximations [23-25] for the exchange-correlation functional.

Solution method:
Ab initio molecular dynamics on the Born-Oppenheimer surface is implemented by a two step method. In the first step the Kohn-Sham equation [26] is solved self-consistently to obtain the electron ground state and the forces on the nuclei. In a second step these forces are used to integrate the equations of motion for the next time step. The calculation of the total-energy and the Kohn-Sham operator in a plane- wave basis-set is done by the momentum-space method [27]. To solve the Kohn-Sham equation the package fhi96md employs the iterative schemes of Williams and Soler [28] and Payne et al [29]. We use the frozen-core approximation and replace the effect of the core electrons by norm- conserving pseudopotentials [30-33} in the fully separable form [34]. The equations of motion of the nuclei are integrated using standard schemes in molecular dynamics such as the Verlet algorithm. Optionally an efficient sructure optimization can be performed by a second order algorithm with a damping term.

Restrictions:
Only pseudopotentials with s-, p-, and d-components are implemented. For highly correlated systems (e.g. f-electrons) the treatment of the exchange-correlation interaction is not appropriate. Relativistic effects are included via scalar relativistic pseudopotentials. The system is assumed to be non-magnetic, but a generalization of the program to magnetic states is straight forward.

Running time:
The test run took 6 min.

References:
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[7] J.A. Alves, J. Hebenstreit and M. Scheffler, Phys. Rev. B 44, 6188 (1991).
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[17] N. Moll et al, Phys. Rev. B 52, 2550 (1995).
[18] A. Gross, M. Bockstedte and M. Scheffler, submitted to Phys. Rev. Lett.
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