SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms) is both a method and its computer program implementation, to perform electronic structure calculations and ab initio molecular dynamics simulations of molecules and solids.

Its main characteristics are:

• It uses the standard Kohn-Sham selfconsistent density functional method in the local density (LDA-LSD) or generalized gradient (GGA) approximations.
• Uses norm-conserving pseudopotentials in its fully nonlocal (Kleinman-Bylander) form.
• The basis set is a very general and flexible linear combination of numerical atomic orbitals (LCAO). It allows arbitrary angular momenta, multiple-zeta, polarized and off-site orbitals.
• 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.
• Besides the standard Rayleigh-Ritz eigenstate method, it allows the use of localized linear combinations of the occupied orbitals (valence-bond or Wannier-like functions), making the computer time and memory scale linearly with the number of atoms. Simulations with several hundred atoms are feasible with modest workstations.
• It is written in Fortran 90. In particular, memory is allocated dynamically, so there is no need to recompile the program when the problem size changes.
• It may be compiled for serial or parallel execution (under MPI). (Note: This feature might not be available in all distributions.)

It routinely provides:

• Total and partial energies.
• Atomic forces.
• Stress tensor.
• Electric dipole moment.
• Atomic, orbital and bond populations (Mulliken).
• Electron density.

And also (though not all options are compatible):

• 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.
• Band structure.