1Departments of Physics and Materials, Imperial College London, Exhibition Road, London SW7 2AZ, UK
2School of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK
3Theory of Condensed Matter, Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, UK
ONETEP is a linear-scaling code for performing first-principles total-energy calculations within density-functional theory (DFT). The method is based on the density-matrix formulation of DFT and involves the iterative minimisation of the total energy with respect to a set of local orbitals and a density kernel. An overview is given of the kernel optimisation methods proposed in the literature and implemented in ONETEP, focussing in particular on the constraints of compatibility, idempotency and normalisation that must be applied. A method is proposed for locating the chemical potential which may be useful in applying the normalisation constraint and analysing the electronic structure near the Fermi level.