Basis Set Generation (PAO)

Siesta uses Pseudo-Atomic Orbitals (PAOs), which are numerical, localized basis functions, to represent the electronic wavefunctions. The quality of the basis set is crucial for accuracy.

• PAO.BasisSize: A predefined recipe for the basis set size. 'SZ' (Single-Zeta) is minimal and fast. 'DZP' (Double-Zeta Polarized) is a good balance of accuracy and cost for most applications.
• PAO.EnergyShift: A key parameter that controls the confinement of the orbitals. A small value (e.g., 0.01 Ry) provides a good balance between localization and completeness.
• PAO.SplitNorm: In split-valence basis sets (like DZP), this parameter determines how the second zeta orbital is generated from the first, affecting the basis set's flexibility.