The use of linear combination of numerical atomic orbitals makes SIESTA a flexible an efficient DFT code. SIESTA is able to produce very fast calculations with small basis sets, allowing computing systems with a thousand of atoms. At the same time, the use of more complete and accurate bases allows to achieve accuracy comparable to those of standard plane waves calculations, still at an advantageous computational cost.
The characteristics of DFT code SIESTA are:
- It uses the standard Kohn-Sham self-consistent density functional method in the local density (LDA-LSD) or generalized gradient (GGA) approximations.
- It uses norm-conserving pseudopotentials in their fully nonlocal (Kleinman-Bylander) form.
- It uses atomic orbitals as a basis set, allowing unlimited multiple-zeta and angular momenta, polarization and on-site orbitals. Finite-support basis sets are the key for calculating the Hamiltonian and overlap matrices in O(N) operations.
- It 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 95 and memory is allocated dynamically. It may be compiled for serial or parallel execution (under MPI).