SIESTA is both a method and its computer program implementation, to perform efficient electronic structure calculations and ab initio molecular dynamics simulations of molecules and solids. SIESTA's efficiency stems from the use of a basis set of strictly-localized atomic orbitals. A very important feature of the code is that its accuracy and cost can be tuned in a wide range, from quick exploratory calculations to highly accurate simulations matching the quality of other approaches, such as plane-wave methods.

The possibility of treating large systems with some first-principles electronic-structure methods has opened up new opportunities in many disciplines. The SIESTA program is open source and has become quite popular, being increasingly used by researchers in geosciences, biology, and engineering (apart from those in its natural habitat of materials physics and chemistry). Currently there are several thousand users all over the world, and the paper describing the method (J. Phys. Cond. Matt. 14, 2745 (2002)) has received more than 8000 citations so far.

SIESTA's main characteristics are:- It uses the standard Kohn-Sham self-consistent density functional method in the local density (LDA-LSD) or generalized gradient (GGA) approximations. Recent versions implement a functional capable of describing van der Waals interactions.
- It employs norm-conserving pseudopotentials in their fully nonlocal (Kleinman-Bylander) form.
- It uses atomic orbitals with finite support as a basis set, allowing unlimited multiple-zeta and angular momenta, polarization and off-site orbitals. Finite-support basis sets are the key for calculating the Hamiltonian and overlap matrices in O(N) operations.
- Projects the electron wavefunctions and density onto a real-space grid in order to calculate the Hartree and exchange-correlation potentials and their matrix elements.

- Total and partial energies.
- Atomic forces.
- Stress tensor.
- Electric dipole moment.
- Atomic, orbital and bond populations (Mulliken).
- Electron density.
- Geometry relaxation, fixed or variable cell.
- Constant-temperature molecular dynamics (Nose thermostat).
- Variable cell dynamics (Parrinello-Rahman).
- Spin polarized calculations (collinear or not).
- k-sampling of the Brillouin zone.
- Local and orbital-projected density of states.
- COOP and COHP curves for chemical bonding analysis.
- Dielectric polarization.
- Vibrations (phonons).
- Band structure.