Pople bases

from Wikipedia, the free encyclopedia

The Pople bases are basic sets used in computational chemistry that use the same radial function for the s and p orbital. The inner atomic orbitals (AOs) are represented by STO-NG basis sets while the valence orbitals are represented by a split- valence basis . This composition leads to increased radial flexibility compared to the pure STO-NG basic sets.

nomenclature

The general designation of the basic sets is X-YZG. The X stands for the number of primitive Gaussians ( primitive Gaussian functions ) that comprise each core atomic orbital basis function . Y and Z indicate that the valence orbitals are each composed of two basis functions. The first basis function consists of a linear combination of Y primitive Gaussian functions. The second basis function accordingly consists of a linear combination of Z primitive Gaussian functions. The use of linear combinations is necessary because the primitive Gaussian functions in the vicinity of the nucleus show a different behavior than atomic orbitals which are represented by Slater orbitals ; this error is minimized by the linear combination. In this case, the presence of two numbers after the hyphen implies that this base set is a split valence base set.

A so-called split-valence basis set uses a contracted Gaussian function for atomic orbitals of the inner shell, but the valence orbitals are characterized by two contracted Gaussian functions. Put simply, it is a combination of a minimal and a double zeta basic set.

6-31G basic set

The 6-31G basic set is certainly one of the most widely used basic sets. Six primitive Gaussian functions form (or contract to) the basic function for the core orbitals. The valence orbitals are formed (or contracted) from (3 + 1) primitive Gaussian functions; the first basic function is made up of the linear combination of three primitive Gaussian functions while the second basic function of the valence orbitals is formed from a primitive Gaussian function. The primitive Gaussian functions used are Gaussian orbitals .

additions

  • * / ** (after the G , e.g. 6-31G * or 6-31G ** ) indicate the use of additional polarization functions (with d symmetry). An asterisk means that all atoms except hydrogen receive polarization functions; Two asterisks mean that all atoms including hydrogen have polarization functions.
  • + / ++ (before the G , e.g. 6-31 + G or 6-31 ++ G ) indicate the use of additional diffuse orbitals of the s and p types. They are necessary for the description of electron-rich systems and anions.

The described additions are to be read in exactly the same way for other basic sentences.

Usage today

Pople's basis sets are a bit out of date, since correlation-consistent or polarization-consistent basis sets typically give better results for similar resources. When using them, it should be noted that some Pople bases have serious defects that can lead to incorrect results.

Individual evidence

  1. ↑ a b c d e Kunz, Roland W .: Molecular Modeling for Users: Application of Force Field and MO Methods in Organic Chemistry . Teubner, Stuttgart 1991, ISBN 3-519-03511-1 , pp. 123-124 .
  2. ^ Aron J. Cohen, Paula Mori-Sánchez, Weitao Yang: Challenges for Density Functional Theory . In: Chemical Reviews . tape 112 , no. 1 , December 22, 2011, ISSN  0009-2665 , p. 289-320 , doi : 10.1021 / cr200107z ( acs.org [accessed July 4, 2018]).
  3. ↑ Basic sets | Gaussian.com. Retrieved July 4, 2018 (American English).