SHELL

Shell user manual is here.

SHELL is a NEW, GENERAL free energy minimization program for solids. It uses lattice statics and quasiharmonic lattice dynamics to calculate analytically the free energy of a crystal, AND its derivatives with respect to the dimensions of the unit cell (the external coordinates) AND the positions of the ions within the unt cell (the internal coordinates).

This is the first program to be able to perform efficient fully dynamic structure optimisation of unit cells of ionic and polar solids containing hundreds of ions.

Why lattice dynamics? Although neglected in recent years, lattice dynamics can be remarkably robust at elevated temperatures, while below the Debye temperature it scores importantly over classical Monte Carlo and molecular dynamics in that it takes account of zero-point energy and other quantum effects. It's cheap and precise, the results are easily interpreted and supply complementary information to that from MD. It also provides a very sensitive test for interatomic potentials.

User functionality

The central capabilities of the program are calculation of the free energy and its strain derivatives. These can be used to determine:

Optimised geometries of unit cells at constant pressure or volume, at any specified temperature and pressure. Optimisation also can be carried out keeping any external or internal coordinate fixed.
Internal energies, free energies and heat capacities,
Phonon dispersion curves and vibrational densities of states.
Thermodynamic Gruneisen functions (see the various bibles by Barron), and mode Gruneisen functions at any wave vector.
Gradient of the free energy with respect to lattice parameters and/or internal coordinates.
Elastic stiffnesses, generated in the static case by analytic calculation of the Hessian and in the dynamic case by numerical differentiation of the gradient.

The optimisations can be carried out either with respect to the lattice parameters (a,b,c,alpha,beta,gamma according to the Bravais lattice type) and EITHER the full set of internal basis coordinates OR a set of internal-coordinates to take advantage of symmetry. For example, for rutile, by symmetry, we require only two lattice parameters a and c to describe the shape of the tetragonal unit cell and one coordinate to parametrise the basis atom positions.

To make the program easy to use, some external utilities are supplied for manipulation of Shell's input and output files. These include:

Determining a set of symmetric internal coordinates given the full set of basis atoms
Calculation of bond lengths and angles
Conversion between units (kJ/mol and eV/? for example - useful both for project students and their supervisors)
Generation of a VRML (3d file format for visualisation) file for the unit cell.
Plotting the forms of short-ranged potential vs. interatomic separation.

An HTML users guide is available (or in postscript for printing out).
There is also an example input file and corresponding output file.

Program availability

Email Dr. Allan or Dr. Barrera .

Selected References

(note that some of the downloadable files are restricted access because of copyright restrictions).

Full details of the theory explained simply:
Free energy derivatives and structure optimisation within quasiharmonic lattice dynamics, M.B. Taylor, G.D. Barrera, N.L. Allan and T.H.K. Barron, Phys. Rev. B 56, 14380-14390 (1997)
(download postscript or PDF file).

The program:
SHELL : A code for lattice dynamics and structure optimisation of ionic crystals, M.B. Taylor, G.D. Barrera, N.L. Allan, T.H.K. Barron and W.C. Mackrodt, Comput. Phys. Commun. 109, 135-143 (1998)
(download postscript).

Some more theory:
The zero static internal stress approximation in lattice dynamics and the calculation of isotope effects on molar volumes, N.L. Allan, T.H.K. Barron and J.A.O. Bruno, J. Chem. Phys., 105, 8300-8303 (1996).
(download postscript or PDF).

Application to defects:
The free energy of formation of defects in polar solids, M.B. Taylor, G.D. Barrera, N.L. Allan, T.H.K. Barron and W.C. Mackrodt, Faraday Discuss., 106, 377-387 (1997)
(download postscript or PDF).

Applications to ionic solids and phase transitions:
Ionic solids at elevated temperatures and high pressures: MgF₂, G.D. Barrera, M.B. Taylor, N.L. Allan, T.H.K. Barron, L.N. Kantorovich and W.C. Mackrodt, J. Chem. Phys. 107, 4337-4344 (1997)
(download postscript or PDF).

Lithium oxide and superionic behaviour - a study using potentials from periodic ab initio calculations, R.M. Fracchia, G.D. Barrera, N.L. Allan, T.H.K. Barron and W.C. Mackrodt, J. Phys. Chem. Solids, 59, 435-445 (1998) (at last! - the publishers lost the manuscript.....)

Application to mixtures and disordered systems:
Hybrid Monte Carlo and lattice dynamics simulations: the enthalpy of mixing of binary oxides, J.A. Purton, J.D. Blundy, M.B. Taylor, G.D. Barrera and N.L. Allan, Chem. Comm., 627-628 (1998)
(download postscript or PDF).

Overview:
Ionic solids at high temperatures and pressure: ab initio, lattice dynamics and Monte Carlo studies, N.L. Allan, G.D. Barrera, C.E. Sims, M.B. Taylor and W.C. Mackrodt, J. Chem. Soc., Faraday Trans, Feature Article (coming soon!).

barrera@unpata.edu.ar, Last update December 1998