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Manuscript Title: Two subroutines for calculating lattice sums and the distortion field due to a point force in hexagonal systems.
Authors: W. Maysenholder
Program title: HEXALAT
Catalogue identifier: ACKQ_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 24(1981)89
Programming language: Fortran.
Computer: HONEYWELL BULL 66/80 DPS.
Operating system: GCOS.
RAM: 23K words
Word size: 36
Keywords: Solid state physics, Hexagonal systems, Elasticity theory, Continuum Green's function, Lattice sums, Location and diffusion Of interstitials, Crystal structure, Defect.
Classification: 7.1, 7.8.

Nature of problem:
In solid state theory the calculation of lattice sums in a perfect as well as in a distorted lattice is frequently required. HEXALAT contains two subroutines which can be used for calculations of this kind in the case of hexagonal systems. The subroutine ARRANGE generates the coordinates of a roughly spherical arrangement of atoms occupying the regular sites of an hcp structure. The coordinates are all stored in one array and can be used for calculating lattice sums. The subroutine DISPL calculates the displacement of an atom due to a double force tensor centred at the origin of the coordinate system in the elastic approximation. As an example, HEXALAT calculates the potential energy of an interstitial at an octahedral site both in the undistorted and in the distorted hexagonal lattice. The interaction potential between the interstitial and one atom is assumed to be given. Calculations of this kind are essential for the determination of the most stable sites for an interstitial and for the investigation of the diffusive behaviour.

Solution method:
The procedure used by ARRANGE is elementary and described in detail in the long write-up. DISPL uses the analytic expressions for the derivatives of the continuum Green's function. The parameters needed by DISPL depend on the elastic constants and are supplied by the subroutine PARA.

Restrictions:
HEXALAT is written in single precision. DISPL is restricted to the evaluation of displacements due to a diagonal double force tensor P with P11=P22 (which holds for point defects at usual interstitial sites).

Running time:
The execution time depends on the number of atoms in the arrangement. The test run (180 atoms) on the Honeywell Bull 66/80 DPS took 2.1 s for compilation and 2.0 s for execution.