| 3:44:31 PM PST - Thu, Dec 10th 2015 |
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Thank you very much for your reply.
I have put the H2 molecules 3A apart and rerun the simulations:
h2.png
With sto-2g basis set there is no appreciable charge oscillation; i guess this (very) small basis set does not fill the space between the 2 molecules, so they are effectively isolated and no charge tranfer can occur. 3-21g and 6-31g predict some charge transfer, but amplitude of charge oscillations is still small. Basis sets with diffuse functions, as well as the larger Dunning basis sets cc-pVDZ and cc-pVTZ show full oscillations.
Charge oscillation frequencies are lower than the previous example, as expected for the increased separation. Generally, for small separation distances, the small and spatially confined basis sets perform relatively well, while for larger separation distances the bigger, more extended basis sets are required.
So, i do see a pattern in the above results; however, lacking any experimental data to compare with, i cannot figure out how one could reliably determine the best basis set for a specific system. Obviously, as seen before, the larger, more extended basis set is not always the best choice. Is there anything that can be done to minimize the basis set dependence?
Thanks again,
Andreas
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