CASTEP

CASTEP

CASTEP is a commercial (and academic) software package which uses density functional theory with a plane wave basis set to calculate electronic properties of solids from first principles.

The CASTEP programme is a first principlesquantum mechanical ("ab initio") code for performing electronic structurecalculations. Within the density functional formalism it can be usedto simulate a wide range of materials including crystalline solids,surfaces, molecules, liquids and amorphous materials; the propertiesof any material that can be thought of as an assembly of nuclei andelectrons can be calculated with the only limitation being the finitespeed and memory of the computers being used.

CASTEP has been completely written for use on parallel computers byresearchers at the Universities of York, Durham, St. Andrews, Cambridge and Rutherford Labsand a new modular Fortran 90 codewritten.

CASTEP is a fully featured first principles code and as such itscapabilities are numerous. Aiming to calculate any physical propertyof the system from first principles, the basic quantity is the totalenergy from which many other quantities are derived. For example thederivative of total energy with respect to atomic positions results inthe forces and the derivative with respect to cell parameters givesstresses. These are then used to perform full geometry optimisationsand possibly finite temperature molecular dynamics. Furthermore,symmetry and constraints (both internal and external to the cell) canbe imposed in the calculations, either as defined by the user, orautomatically using in-built symmetry detection.

Theory and Approximations

Starting from the many body wavefunction of the time, nuclear and electronic coordinates an adiabaticapproximation is made (the Born-Oppenheimer approximation).The code also makes use of Bloch's Theorem which means a wavefunction of a periodic system has a cell-periodic factor and a phase factor. The phase factor is represented by a plane wave. From the usage of Bloch's Theorem, it would be ideal to write the wavefunction in plane waves for the cell-periodic factor and the phase factor. From this the basis functions are orthogonal and it would be easy to Fourier transform from real to reciprocal space and vice versa.Fast Fourier Transforms are used throughout the CASTEP code and so is the Ewald summation method for Coulombic energies etc.Along with plane waves and some form of minimisation(conjugate gradient) , pseudopotentials are essential to the CASTEP code for reducing the calculation cost. Pseudopotentials replace the atomic nucleus and the core electrons by an effective potential.

See also

* Quantum chemistry computer programs

External links

* [http://www.castep.org/ CASTEP official website]


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