Dear Mustafa,

BSSE is an acronym for "Basis Set Superposition Error". BSSE can occur when calculating reaction energies. For example in the A + B = AB calculation, the energy of AB could be lower because B's basis sets may lower the A part (on the right hand side) and of course will not change the energy of A on the left hand side.

If one worries about this error, the solution in Spartan is to do all three calculation (A, B, and AB) at a higher basis set. Typically this BSSE correction energy is small for large basis sets. Often smaller than other neglected terms such as incomplete basis, neglect of electron correlation, infinite gas phase, approximate geometries and zero-point energy.

Another well known way of dealing with BSSE is the 'counter poise' method. In this approach the calculations on the left hand side'A' and 'B' have included in them the basis functions of 'B' and 'A' added respectively. Unfortunately this method is geometry dependent and assumes the geometry of A and B do not change much when going from A and B to AB.

Spartan's preferred way of dealing with BSSE is to do a single point energy with a large basis set using the "dual-basis" approximation. This is typically more accurate than the usual small basis-set counter poise' method, includes a correction to finite basis set size, and is much quicker than the larger basis set calculation.

As an example of the affect of different basis sets the first table in the following link summarizes the results for a HF dimer of water. The dimer was optimized at for each basis set:

https://www.wavefun.com/support/sp_compfaq08/Energy_FAQs.html

Hoping this will be helpful,

Rafik

Dear Rafik,

there are some misunderstandings in your answer. Please let me clearify a bit.

BSSE has not really much to do with an reaction. It is present in all calculations with finite basis sets. Intermolecular and intramolecular.

It most significant implication can be found in non-bonded systems (intermolecular). For these systems counter poise (CP) correction is very much advisable.

Further more. Please note, that CP corrections can not only be applied to energies, but also gradients. Thus, CP corrected structures can be optimized. 

To Mustafa's question. The smaller the value of BSSE gets, the less defective the energies without CP correction would be.

Dear Arnim,

Please read the publication contained in the link depicted in my first answer.

Rafik

Dear Rafik,

I am aware that you copy-and-pasted your answer from the internet, but this does not make it any better.

Dear Armin,

Please note that I know every word written in the publication. This is my field. I gave you the reference for you to look at it. You call it internet; the reality this is more than an internet. Please go to the following references that were cited in the "INTERNE:. If you have some thing against these publications, you can write a paper, editorial or any publication in which you show the incorrect information depicted in these publication.

References:

"Gaussian-3 (G3) theory for molecules containing first and second-row atoms" J. Chem. Phys. Vol. 109, No. 18, (Nov. 8 1998) pp 7764-7776

"Gaussian-3 theory using reduced Moller-Plesset order" J. Chem. Phys. Vol 110, No. 10, (Mar. 8 1999) pp 4703-4709

"A Brief Guide to Molecular Mechanics and Quantum Chemical Calculations" W.J. Hehre, J. Yu, P.E. Klunzinger, L. Lou; Wavefunction Inc.

"A Guide to Molecular Mechanics and Quantum Chemical Calculations" W.J. Hehre; Wavefunction Inc. (2003)

As of June 2003 this books ships with all versions of Spartan and Spartan Essential.

"Ab Initio Molecule Orbital Theory", Warren J. Hehre, Leo Radom, Paul v.R. Schleyer, John A. Pople; John Wiley & Sons, Inc. (1986)

"1998 CODATA Recommended Values of the Fundamental Physical Constants"

which derives the "..values of the basic constants and conversion factors of physics and chemistry resulting from the 1998 least-squares adjustment of the fundamental physical constants as ... published by the CODATA Task Group on Fundamental Constants and as recommended for international use by CODATA. The new, 1998 CODATA set of recommended values replaces its predecessor published by the Task Group and recommended for international use by CODATA in 1986..." and prior to that in 1973.

Rafik

Dear Rafik,

I made three points of clearification to the information from the FAQ website of Wavefunction Inc., which you have cited. (it is not a real publication, though)

If you have any question to the points I made, I will be happy to discuss them with you. If you do not want to discuss that, then don't. But do not just repeat your initial post.

And why would you cite these reference? Most of them have nothing to do with BSSE. Why would you think that this strengthen your point of view?

Arnim

Dear Armin,

Mustafa's question was:  large or small value of BSSE energy refers to what? I gave him the meaning of the term and the interpretation of its value.  Additionally the link from the INTERNET!!!!!!!!! was to show Mustafa some examples along with BSSE values.

Anyway thanks for your comments, maybe someday they will be helpful,

Rafik

Dear Mustafa, the basis set superposition error (BSSE) is related to the basis set and the method used to study situations where there are involved fragments. The BSSE is small when a large basis set is used, in fact, if the basis set contains an infinity number of functions then the BSSE will be zero. Also it is well recognized that Hartree-Fock and KS methods induce small BSSE, since these methods use only occupied orbitals. However, when unoccupied orbitals are involved in the evaluation of the total energy then these virtual orbitals have a large impact on the BSSE. Best regards. Jorge