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Manuscript Title: Ab initio pseudopotentials for electronic structure calculations of poly-atomic systems using density-functional theory.
Authors: M. Fuchs, M. Scheffler
Program title: fhi98PP
Catalogue identifier: ADKA_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 119(1999)67
Programming language: Fortran.
Computer: IBM/RS 6000.
Operating system: UNIX.
RAM: 1M words
Word size: 32
Keywords: Solid state physics, Band structure, Pseudopotential, Total energy, Electronic structure, Density functional, Local density, Generalized gradient.
Classification: 7.3.

Nature of problem:
The norm-conserving pseudopotential concept allows for efficient and accurate ab initio electronic structure calculations of poly-atomic systems. The key features of this approach are: (i) Only the valence states need to be calculated. The core states are considered as chemically inert, and removed within the frozen-core approximation. This exploits that many chemical and physical processes are governed by the valence states but only indirectly involve the core states. (ii) The valence electrons move in a pseudopotential which is much smoother than the true potential inside the small core regions around the nuclei, while reproducing it outside. This pseudopotential acts on smooth pseudo wavefunctions that are equivalent to the true valence wavefunctions, but avoid the radial nodes that keep the true valence and core orbitals orthogonal. This enables the use of computationally expedient basis sets like plane waves, and facilitates the numerical solution of the Schrodinger and Poisson equations in complicated systems. (iii) The norm-conservation constraint ensures that outside the core the pseudo wavefunctions behave like their all-electron counterparts over a wide range of different chemical situations. Along with a proper design, this makes the pseudopotential approach a dependable approximation in describing chemical bonds.
Derived and applied within density-functional theory [1-3], norm-conserving pseudopotentials [4-6] enable total-energy calculations of complex poly-atomic systems [7-9] for a multitude of elements throughout the periodic table. Questions addressed with pseudopotentials provided by this code, or its earlier version, range from phase transitions [10,11], defects in semiconductors [12-14], the structure of the diffusion on surfaces of semiconductors [15-17], simple metals [18], and transitions metals [19-21], up to surface reactions [22,23], including molecules [24,25] of first-row species.
This package is a tool to generate and validate norm-conserving pseudopotentials, usable either in semilocal or in fully separable form, and including relativistic effects. Exchange and correlation is treated in the local-density approximation based on Ceperley and Alder's data [26] as parametrized, e.g., by Perdew and Wang [27], or in the generalized gradient approximation, as proposed by Perdew, Burke, and Ernzerhof (PBE) [28], Perdew and Wang (PW91) [29], Becke and Perdew (BP) [30,31], and by Lee, Yang, and Parr (BLYP) [32].

Solution method:
The first part of the program (psgen) generates pseudopotentials of the Hamann [33] or the Troullier-Martins type [34], based on a scalar-relativistic all-electron calculation of the free atom. A partial core density can be included to allow for nonlinear core-valence exchange-correlation [35] where needed, e.g., for spin-density functional calculations, alkali metal compounds, and the cations of II-VI compounds like ZnSe. The second part (pswatch) serves to assess the transferability of the pseudopotentials, examining scattering properties, excitation energies, and chemical hardness properties of the free pseudo atom. Transcribing the pseudopotentials into the fully separable form of Kleinman and Bylander [36], we verify the absence of unphysical states by inspection of the bound state spectrum and by the analysis of Gonze et al [37]. The convergence of the pseudo wavefunctions in momentum space is monitored in order to estimate a suitable basis set cutoff in plane wave calculations.

Restrictions:
(i) Only some of the GGA's currently in use are implemented, others may be readily added however. (ii) The present pseudopotentials yield the correct relativistic valence levels where spin-orbit splittings are averaged over, as it is intended for most applications.

Unusual features:
The output is tailored to the graphics software XMGR or XVGR (both are public domain packages) [38].

Running time:
The time for the test run took about 1 min.

References:
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[38] P.J. Turner, ACE/gr User's Manual in Software Documentation Series, SDS3, 91-3 (Oregon Graduate Institute of Science and Technology, Beaverton, 1992); see also the WWW URL http://plasma-gate.weizmann.ac.il/Xmgr.