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Manuscript Title: Simultaneous calculation of the equilibrium atomic structure and its electronic ground state using density-functional theory.
Authors: R. Stumpf, M. Scheffler
Program title: fhi93cp
Catalogue identifier: ACTF_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 79(1994)447
Programming language: Fortran.
Computer: IBM RS/6000 370.
Operating system: UNIX.
RAM: 4000K words
Word size: 32
Keywords: Solid state physics, Density-functional Theory, Local-density, Approximation, Ab-initio Pseudopotentials, Plane-wave basis, Supercell, Chemical binding, Total-energy, Structure optimization, Crystals surfaces, Crystals defects in, Molecules, Crystal field.
Classification: 7.3, 16.1.

Nature of problem:
In poly-atomic systems, such as molecules [1,2], crystals and defects in crystals [3,4], and surfaces [5,6,7,8,9], it is highly desirable to evaluate the electronic structure and to determine the stable as well as metastable atomic geometry from first principles and without introducing sever approximations. For a correct treatment of the chemical binding it is most important to take the quantum-mechanical kinetic-energy operator as well as the self-consistent electronic charge density into account. The main challenge of state-of-the art calculations is to treat systems composed of 100 or more atoms without any restrictions to the system symmetry. The computer code described below enables such calculations, where the only (possibly relevant) approximation is the exchange-correlation energy functional, which is taken in the local-density approximation [10]. We use the frozen-core approximation, treating the ions by ab-initio, fully separable pseudopotentials [11,12].

Solution method:
The momentum-space method [4,13] is the most efficient way to calculate the hamiltonian in a plane-wave basis set. To solve the eigenvalue problem and to achieve self-consistency Car and Parrinello [14] proposed an iterative approach, where in each iteration the Hamilton operator is applied to the wave functions. This gives the analog of a force on each wave-function coefficient which points towards the electronic ground state. The program uses this iterative approach for the electronic wave functions and optimizes the total energy with respect to the atomic- structure degrees of freedom by a damped dynamics, using the Hellman- Feynman forces on the ions.

Restrictions:
Only pseudopotentials with s- or p-non-locality can be used in the present version and they should be given in a separable form, as for example listed in Ref. [12]. The shape and size of the cell may not change during the calculation.

Running time:
Time for test run took 6 min.