Mo/ller–Plesset perturbation investigation of the He2 potential and the role of midbond basis functions

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Abstract
The Mo/ller–Plesset perturbation theory up to the complete fourth order is applied to calculate the ab initio van der Waals energy potential of He2. A scheme of constructing a very efficient basis set is proposed based on theoretical considerations. Such a basis set contains a set of Gaussian functions (usually of low angular momentum) centered at the midbond of the van der Waals molecule, which have been proven very efficient to reproduce the intersystem correlation interaction energy normally to be achieved by use of the nucleus-centered high angular momentum polarization functions. A nuclear-centered moderate set [5s4p2d] augmented by a set of midbond functions such as {3s3p2d} produces a value of 10.04 K for the well depth of He2 at the complete fourth-order (MP4) theory. Results from a series of other basis sets suggest that this value is nearly saturated with the further increase of basis, which is in agreement with the estimate of the complete basis-set limit. Calculations of the potential at the minimum and the other distances (3.0–10.0 a0) confirm that the MP4 method, together with the use of midbond functions, is a reliable and convenient choice for the accurate calculations.