CASTEP is a shared-source academic and commercial software package which uses density functional theory with a plane wave basis set to calculate the electronic properties of crystalline solids, surfaces, molecules, liquids and amorphous materials from first principles.

The name CASTEP was adopted by the new codebase, but without the implied former meaning since the code was now parallel, and capable of computing many quantities besides the total energy.

From this the basis functions are orthogonal and it is easy to perform a Fourier transform from real to reciprocal space and vice versa.

Along with plane waves and iterative diagonalization methods (via conjugate gradient or blocked Davidson algorithms), pseudopotentials are essential to the CASTEP code for reducing the computational expense of the calculation.

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.