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A new computational method within the framework of extended group function models is introduced for describing intermolecular interactions between closed shell systems. The model is size extensive, applicable for any intersystem distance, and has a conceptual structure which facilitates interpretation. The basis set superposition error can be eliminated at the correlation level. Test calculations have been performed on the helium dimer and the helium trimer.

An extended group function model has been applied to determine the three‐body potential for the ground state of the helium trimer. The basis set for the calculations is an uncontracted (17s,6p,4d,3f) set of Gaussian type functions contracted to [8s,6p,4d,3f]. Three different types of configurations were considered: (i) equilateral triangles of side R, R∈{3.5,4.0,...,8.5,9.0} a.u., (ii) linear configurations with R12=R23, R12∈{3.5,4.0,...,8.5,9.0} a.u., and (iii) a set of pseudo‐random configurations. For the equilateral triangular configurations and the linear configurations, and with equal distances larger than 5.5 a.u., the magnitude of the ratio between the three‐body potential and the sum of the corresponding two‐body potentials, is less than 0.003. The value of the three‐body potential for the triangular and the linear configuration with equal distance R=5.669 289 a.u.=3 A, is respectively 0.040 μH and −0.045 μH. In the long range region the calculated three‐body potential is very close to the Axilro...

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+.

A model aimed at describing intermolecular interactions is formulated within the context of extended geminal models. The model has the following properties: It has a conceptual structure which facilitates interpretation. The model can be applied for any intersystem distances. The model is size extensive. Basis set superposition errors can be practically eliminated. The model is computationally efficient. A test calculation on the helium dimer yields a well depth of 10.64 K. The experimental determined value of the well depth is (10.79±0.22) K.

An extended geminal model is derived by applying perturbation theory to a matrix representation of the electronic Hamiltonian. The matrix representation is formed by a complete set of orthogonal configuration functions generated from a given finite orbital basis set. By construction, the function defined by the APSG approximation (antisymmetric product of strongly orthogonal geminals) is an element of this set of configuration functions. Considering the nondiagonal part of the Hamiltonian matrix as the perturbation, our model is obtained by a partitioning and reordering of the infinite order Rayleigh–Schrodinger perturbation expansion. The model has a hierarchical structure, is size extensive, and has favorable properties with respect to interpretation. In a test calculation on the water molecule, it is demonstrated that the extended geminal model truncated at the double pair correction level, recovers 100.8% of the correlation energy defined by the corresponding full CI calculation.

A molecular system is partitioned into fragments. Within the context of extended geminal models, it is shown that the total electronic energy can be written as a sum of intra‐ and interfragment energies. On the basis of this type of analysis, the difference in equilibrium bond angles in H2O and H2S, NH3 and PH3, is discussed.

A scheme for obtaining approximate solutions of the electronic Schrodinger equation is proposed in terms of an extended group function model. The model is formulated within the context of the method of moments. The model is size consistent, and it has additive accuracy since the correction to a pure antisymmetrized product function is obtained as additive terms describing intragroup and intergroup correlation corrections. In a test calculation on the ground state potential energy curve for the F2 molecule, the advocated model yields a dissociation energy of 1.63(1.66) eV and an equilibrium internuclear distance of 1.392(1.412) A. Experimental values are within parenthesis.

Within the framework of the general extended geminal models, two new approximate models are introduced. Compared with previous models, the double pair correction terms {eKL} are given a more accurate representation. The models are tested against full CI calculations on the beryllium atom, the neon atom, and the beryllium dimer.