Background
In the ongoing saga (read part I) with acrolein solvated in water, we were having problems with our high quality embedding potential just not converging nicely when increasing system sizes, QM-region sizes. We were using a functional to get good shielding constants (the KT3 functional) with a basis set designed to get good shielding constants (the apcS-1 basis set) but no matter what tried, the widely adopted TIP3P potential for water just did it better than we did. And we were just confused!
Solution
The problem turned out to be the basis set that with the diffuse functions leaked electron density out into the MM-region when using the PE potential and this was causing some disturbance with the induced dipoles. On the contrary, our belief is that TIP3P is a much too soft potential and there is a large degree of error cancellation because of that. Both are effects I would think to be very small and have no real impact but I had also forgotten that we were talking about nuclear shielding constants which are sensitive as f*ck the wave function.
I switched to the smaller pcS-1 basis set and all that leaking-trouble went away. Everything that we did not expect was gone. TIP3P was converging rather slowly and the PE-model was quickly converging as is shown below for an average of 120 snapshots.
The PE numbers (blue) are converged at QM size 3 but even size 1 and 2 show some good indications. For size 1, acrolein is solvated in purely classical water. As we would expect, the improved potential means that we are converging (fast) towards a value for the absolute shielding of acrolein in water valued -223.2 ppm.
Outlook
We will return to the use of diffuse basis functions and a proper quantum mechanical fix in a later paper as this work here sparked my co-worker +Jógvan Magnus Haugaard Olsen into actually picking up work he had done on repulsion during his Ph.D. (pdf of his thesis)
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