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== Features ==
SIESTA has these main characteristics:
* It uses the standard [[Kohn–Sham equations|Kohn-Sham]] self-consistent [[Density functional theory|density functional]] method in the [[Local-density approximation|local density]] (LDA-LSD) and generalized gradient (GGA) approximations, as well as in a non-local function that includes [[van der Waals interactions]] (VDW-DF).
* It uses norm-conserving [[pseudopotential]]s in their fully non-local (Kleinman-Bylander) form.
* It uses [[atomic orbital]]s as a basis set, allowing unlimited multiple-zeta and angular momenta, polarization, and off-site orbitals. The radial shape of every orbital is numerical, and any shape can be used and provided by the user, with the only condition that it has to be of finite support, i.e., it has to be strictly zero beyond a user-provided distance from the corresponding nucleus. Finite-support basis sets are the key to calculating the Hamiltonian and overlap matrices in O(N) operations.
* Projects the electron wave functions and density onto a real-space grid to calculate the Hartree and exchange-correlation potentials and their matrix elements.
* Besides the standard [[Rayleigh–Ritz method|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).
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