and it is easy to perform a Fourier transform from real to reciprocal space and vice versa. Fast Fourier Transforms are used throughout the CASTEP code, as is the Ewald summation method for Coulombic energies. Along with plane waves and iterative diagonalisation methods (via conjugate gradient or blocked Davidson algorithms), pseudopotentials are essential to the CASTEP code for reducing the computational expense of the calculation. Pseudopotentials replace the atomic nucleus and the core electrons by an effective numeric potential. Geometry optimisation CASTEP is capable of optimising the atomic geometry of a system in several different ways. The default is BFGS, whereby an approximation to the Hessian matrix is built up over successive electronic minimisation steps and used to find a search direction at each. Damped molecular dynamics is also possible and often quick to converge, sometimes even faster than BFGS, due to wavefunction extrapolation. Damped MD is most often chosen over BFGS, however, due to the possibility for non-linear ion constraints. A further alternative is the FIRE scheme, which takes approximately the same approach as damped MD, but based on slightly different methodology. See also Quantum chemistry computer programs References External links Source repository Computational chemistry software Physics software Density functional theory software Science and technology in Cambridgeshire