pseudoorbital

A pseudoorbital is a concept used in computational chemistry, specifically in the context of ab initio and density functional theory calculations. It represents a mathematically constructed orbital that is not a true molecular orbital derived from the linear combination of atomic orbitals (LCAO) approximation. Instead, pseudoorbitals are typically formed from occupied and/or virtual orbitals of a reference system, such as a free atom or a fragment molecule.

The primary purpose of using pseudoorbitals is to improve the efficiency and accuracy of electronic structure calculations, particularly for systems where standard basis sets might be inadequate or lead to numerical instabilities. They can be employed to describe localized electronic features or to provide a more robust description of bonding in complex molecules. Pseudoorbitals can also be useful in embedding calculations, where a small part of a larger system is treated with high accuracy, while the rest is approximated.

While true molecular orbitals are eigenfunctions of the Fock operator or the Kohn-Sham operator, pseudoorbitals are defined based on different criteria, often related to energy localization or spatial confinement. The choice of how to construct pseudoorbitals can vary depending on the specific computational method and the problem being addressed. Their use allows for a more flexible and sometimes more physically intuitive representation of electronic density.