Intermolecular potentials calculated by an extended group function model: Theory

Abstract

A set of computational models, denoted extended group function models, are introduced for describing intermolecular interactions of closed shell systems. The models have the following properties: They have a conceptual structure which facilitates interpretation. The models can be applied for any intersystem distances. The models are size extensive. The basis set superposition error can be eliminated at the correlation level. The models are computationally efficient. A test calculation is performed on the complex Ne⋅Li+.

An accurate computational model for the study of intermolecular interactions

“…1. Here, we sketch briefly the elements which are necessary for the construction of the new computational model.…”

“…Due to the small magnitude of these terms, it seems reasonable to approximate ⑀ KLM by the first nonvanishing term in an appropriate perturbation expansion. This is the approach defined in the work by Ro "eggen et al 1 A slightly more accurate procedure can be introduced by considering a contracted CI model. 19 Since we are considering closed shell systems comprising N electrons pairs, the RHF function, ⌽ RHF , is defined in terms of N spatial orbitals, i.e.…”

“…The BSSE at the correlation level is eliminated by construction. 1,11 At the restricted Hartree-Fock level ͑RHF͒ there is a small BSSE. However, due to the large set of s-type functions adopted in this work, the BSSE at the RHF level, estimated by the Boys-Bernardi counterpoise correction scheme, 38 is less than 0.01 K. There is a small uncertainty in the full CI correction ⑀ 1,2 ͑3͒ .…”

“…Recently we introduced an extended group function model for the study of weak intermolecular interactions, 1 closely related to the extended geminal models. [2][3][4][5][6][7] The extended group function approach shares several attractive features with its predecessors: They yield energy additivity when applied to a complex of isolated subsystems; apply to any intersystem distances; and have a conceptual structure which facilitates interpretation.…”

“…1 Intrasystem correlation energy shall be described by configuration interaction ͑CI͒ based on single and double replacement functions. In addition we introduce a contracted CI technique 19 for describing the triple pair correlation terms.…”