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 11 February 2012

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http://www.castep.org

Introduction

The CASTEP Code.

Calculating the properties of materials fron scratch - First-principles calculations involve no adjustable parameters, are based on an explicit quantum treatment of the electrons in a model system, which means solving Schroedinger's equation to find the electronic ground state. The calculations yield the total energy of assemblies of atoms, and forces on the atoms. This knowledge enables us to calculate a vast range of properties with near chemical accuracy. Density Functional Theory (DFT) is a powerful, general theory relating the total energy of a system of interacting electrons in an external potential to the electron density. Almost all our calculations employ DFT to describe the valence electrons. The great importance of DFT and its application in simulation was recognised by the award of the 1998 Nobel Prize in chemistry to Walter Kohn and John Pople.

Car-Parrinello Methods The DFT plane-wave pseudo potential method, was pioneered in the 80's by Roberto Car and Michele Parrinello. This approach made calculations possible far beyond previous approaches. The methods rely on a plane-wave basis, pseudo potentials and the use of density-functional theory to describe the valence electrons in a model system. Other ingredients include fast-Fourier transforms and minimization of the total energy rather than matrix diagonalization.

The main approximations In Car-Parrinello calculations there are two main approximations. The first is the way in which the exchange-and-correlation energy of the electrons is handled. DFT provides a very general theory involving functionals of the electron density, but does not say what these functionals are, only that they must exist. In practise it has been found that the Local-Density Approximation (LDA), sometimes extended to take account of density gradients (the Generalised Gradient Approximation or GGA) are remarkably accurate, despite their simplicity. The second approximation is the pseudo potential approximation. This is basically a way of avoiding the need to include all the electrons in the calculations. Only the valence electrons are treated explicitly. The core electrons are assumed to be in exactly the same state as in the free atom. The effective interaction between the valence electrons and the ion cores is described by a pseudo potential for each kind of atom. This pseudo potential is generated by first-principles calculations on free atoms.

What you can calculate. The total energy and forces supplied by first-principles calculations can be used to do many things. One of the simplest and most important is the determination of equilibrium structures. It is widely used for complex problems, such as the equilibrium structure of point defects, grain boundaries, surfaces and molecules at surfaces. Closely related to the treatment of equilibrium structures is the calculation of vibrational properties, which is also widely practised. We also use first-principles calculations for dynamical simulations, where the forces on the atoms are used to generate the time evolution of the atomic positions. Dynamical simulations allow the study of solids and liquids in thermal equilibrium, through the calculation of quantities such as thermodynamic functions, radial distribution functions, diffusion coefficients and dynamical structure factors. Even the first-principles calculation of phase diagrams is becoming possible.

Functionality

CASTEPs key features are:

Ultrasoft or norm-conserving pseudopotentials with non-linear core corrections

Complete pseudo potential library

Comprehensive range of minimisation methods: Density Mixing, RM-DIIS, Conjugate Gradients band-by-band and all-bands

Full structural relaxation and molecular dynamics capabilities

Local Density and Generalised Gradient approximations, spin-polarisation

Supported by 100s of successful and published applications

Compatible with Accelrys Cerius2 and Materials Studio user interfaces giving automatic setup and full visualisation facilities

Projects using CASTEP

The computational materials science group use the CASTEP program in their projects and example of which is Band Parallelism in CASTEP: scaling to more than 1000 cores. The CASTEP code is developed in the UK and licensed by over 150 UK academic research groups and scientists, with many more worldwide via sales by the commercial partner Accelrys Inc. The CASTEP code is developed in the UK and licensed by over 150 UK academic research groups and scientists, with many more worldwide via sales by the commercial partner Accelrys Inc. CASTEP was designed from the outset as a parallel code and current releases scale well to several hundred cores. However the extant parallelization schemes do not allow full exploitation of massively-parallel supercomputers with tens of thousands of cores, such as HECToR or its successors. A project, funded by a HECToR CSE grant, to add a further layer of parallelism extends the usable scalability into the thousands of cores regime.Currently CASTEP has three levels of parallelism. For more information see Frontiers 2009 CASTEP

Academic CASTEP suuport from the STFC website can be found at Academic support

Obtain a copy

The UKCP-Accelrys agreement secures the right to the CASTEP source code for all UK academics. UKCP believes this is a remarkable benefit, offering a high-quality code which is the product of many years' effort by academic researchers and software professionals. Even more important though is the opportunity for UK researchers to contribute to the continued development and enhancement of CASTEP.
Daresbury Laboratory distributes CASTEP to UK academics. To obtain the code you will be required to sign a license agreement and provide some details about yourself and your institution. After we receive these the software will be sent to you. Our address is:

Dr Keith Refson
UKCP/CASTEP
STFC Rutherford Appleton Laboratory
Harwell Science and Innovation Campus
Didcot
OX11 0QX
E_mail: enquiries@stfc.ac.uk

Academic release 4.2 is now available for distribution. Please write to the above address, clearly marking your letter or e_mail "UKCP/CASTEP", to obtain the requisite forms. Once we receive these completed and in order the software will be posted to you. Please allow reasonable time for delivery!
Please note: at present this agreement covers academic researchers in the UK only, and we regret that we cannot distribute CASTEP to researchers in other countries. Electronic versions of the CASTEP paperwork are also available via e-mail.