SIESTA

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. SIESTA uses strictly localized basis sets and the implementation of linear-scaling algorithms. 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's backronym is the Spanish Initiative for Electronic Simulations with Thousands of Atoms. Since 13 May 2016, with the 4.0 version announcement, SIESTA is released under the terms of the GPL open-source license. Source packages and access to the development versions can be obtained from the DevOps platform on GitLab. The latest version Siesta-4.1.5 was released on 4 February 2021. Features SIESTA has these main characteristics: It uses the standard Kohn-Sham self-consistent density functional method in the 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 pseudopotentials in their fully non-local (Kleinman-Bylander) form. It uses atomic orbitals 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 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).