[GAUSSIAN] Energy catastrophe in PES bond length scan?

01 June 2022 0 2K Report

Hi,

I'm trying to model the potential energy surface of the N-H bond length in NH4+ spanning a bond length from, say, 0.6 Å to 10 Å to study the behavior of the potential energy as well as partial charges as the proton departs. I do this using flexible scanning in Gaussian 16. I have noticed a lot of weird behavior here:

When I try simple RHF/6-311+G(d,p), I get a seemingly optimum PES curve resembling the one from Morse potential. However, when I try including electronic correlation via MP2, B3LYP, or even CCSD(T), sometimes with aug-cc-pVTZ, I always get an energy catastrophe where, after bond breakage, the potential energy of the system goes down toward negative infinity as the "bond" length increases.

In both RHF and higher levels, the Mulliken partial charge on the departing proton never reaches +1. The maximum it gets is +0.8 even at distances of 30 A.

Except for RHF, I always noticed that the remaining NH3 fragment would lose its trigonal pyramidal geometry as H+ goes away, and becomes trigonal planar. which is counterintuitive.

On the contrary, none of this weird behavior was noticed when I did the same type of calculation on H3O+. Any clues?

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