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

Accuracy and speed can be set in a wide range, from quick exploratory calculations to highly accurate simulations matching the quality of other approaches, such as the plane-wave and all-electron methods.

SIESTA can produce very fast calculations with small basis sets, allowing the computation of systems with thousands of atoms.

Alternatively, the use of more complete and accurate bases achieves accuracies comparable to those of standard plane wave calculations, with competitive performance.

Since its implementation, SIESTA has been used by researchers in geosciences, biology, and engineering (extending beyond materials physics and chemistry) and has been applied to a large variety of systems including surfaces, adsorbates, nanotubes, nanoclusters, biological molecules, amorphous semiconductors, ferroelectric films, low-dimensional metals, etc.