Canonical purification of the density matrix in electronic-structure theory

Two methods are suggested for performing ground-state electronic-structure calculations within the independent-electron approximation. Both methods in volve the purification of a specified initial density matrix, which can be done either canonically (at a fixed electron count N-e) or grand canonicall y (at a fixed chemical potential mu). Linear system-size scaling is achieve d either way, as we illustrate in example tight-binding calculations on car bon nanotubes. [S0163-1829 (98)09243-1].