The description of weakly bound systems in electronic structure calculations can be improved by placing a small number of basis functions in the region between atoms/molecules. The so-called midbond functions presented below have been optimised for use alongside correlation consistent basis sets.
Further details can be found in the Molecular Physics article.
Please select the midbond functions you'd like to download:
As mentioned above, midbond functions can greatly improve the description of weakly bound systems. They can also avoid numerical difficulties caused by the use of very diffuse atom-centred functions. Much of the work in this area stems from the contribution of Tao and Pan, who investigated the use of midbond functions for He2. The midbond functions of Tao and Pan, which were "briefly optimized for intersystem correlation interaction energy" are available to download above. But, Tao and Pan suggest that the interaction energy is not sensitive to the exponent values or their position, but we find that by fully optimising the exponents it is possible to use a considerably smaller set of midbond functions than those of Tao and Pan without loss of accuracy.
Midbond functions can usually be incorporated into a quantum chemical calculation by positioning a dummy atom exactly halfway between the interacting fragments. The dummy atom should have no nuclear charge or electrons but possesses basis functions (some packages term these ghost atoms, check the documentation).
The combination of midbond functions and explicitly correlated methods can be particularly effective for calculating interaction energies, yet requires the specification of three auxiliary basis sets (ABSs) for the midbond functions. We have developed universal ABSs for midbond functions that can be downloaded as JKFit, MP2Fit and OptRI above (matching the MOLPRO terminology for fitting of Fock and exchange matrices, two-electron electron repulsion integrals, and many electron integrals, respectively).