  We have developed and implemented a self-consistent density functional method
using standard norm-conserving pseudopotentials and a flexible, numerical LCAO
basis set, which includes multiple-zeta and polarization orbitals. Exchange and
correlation are treated with the local spin density or generalized gradient
approximations. The basis functions and the electron density are projected on a
real-space grid, in order to calculate the Hartree and exchange-correlation
potentials and matrix elements, with a number of operations that scales
linearly with the size of the system. We use a modified energy functional,
whose minimization produces orthogonal wavefunctions and the same energy and
density as the Kohn-Sham energy functional, without the need of an explicit
orthogonalization. Additionally, using localized Wannier-like electron
wavefunctions allows the computation time and memory, required to minimize the
energy, to also scale linearly with the size of the system. Forces and stresses
are also calculated efficiently and accurately, thus allowing structural
relaxation and molecular dynamics simulations.
