Ghost orbitals and the basis set extension effects

✍ Scribed by N.S. Ostlund; D.L. Merrifield

• Publisher: Elsevier Science
• Year: 1976
• Tongue: English
• Weight: 305 KB
• Volume: 39
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(76)80343-0

✦ Synopsis

The use of identical basis sets for monomer and dimer, in intermolecular ealcuhtions, is discussed. Evidence is presented that such a technique eliminates a basis set extension effect and gives a considerably better description of the intermolecular interaction than normal supermolecule calculations.

Wc wish to report here a dramatic illustration of the basis set extension effect* and the results of a practical procedure which avoids it.

The ab mitio calculation of intermolecular interactions is a subject of considerable current interest.

-When wavefunctions for monomers A and B overIap strongly, long-range theories are inappropriate, and a majority of_calculations use the "supermolecule" approach, i.e. they treat the dimer Al3 as one large molecule. The change in any property P as a function of the intermolecular distance* R, is the calculated property of the dimer AB, minus the sum of the calculated properties of the two monomers, AP(R) = P,&R)-PA-Ps-The basis set for the dimer calculation is the union of the basis sets of the two monomers.

A serious problem with this approach is the basis set extension effect; if the monomer basis sets are incomplete and, particularly, if they are smah, then -as the two monomers approach, the orbit& on B improve the monomer properties of A, the orbit& on A improve the monomer properties of B, and one obtains a poor representation of the interaction. Thii effect may be particularly disasterous, by virtue of the variational principle, if the interaction energy is * The effec; is probably familiar to many researchers in the field. It appears to have been most recently discussed in -ref. [l]. :