HJ: During the opt NH2 is being pulled in by the SO2 group and Sulfur breaks the tetrahedral structure and the distance between the oxygens and NH2 group is not chemically intuitive. This was not the case with SIMIRNOFF99Frost. This was introduced in parsley due to bad parameters. The parameters involved are a30, a31, the values moved far away in 1.2 and 1.3 when compared to SMIRNOFF99Frost. The possible need of higher order potential terms in angle terms. Long back when we talked about sp2 carbon’s sum of angles going over 360 and discussed about a higher order term. CB also suggested a basic check of sulfonamide or hypervalent nitrogen containing molecules similar to HMR canary test.
BS: Is it for the gas phase too
HJ: Yeah.
CB: Thank you HJ for digging into this. First-aid to stop the bleeding, even if it is not perfect, is to go back to 1.3.0 and correct the equilibrium angle for the tetrahedron to 109.5 from 99.1. First thing is to find how and where this is happening? How can we prevent it from happening? Something has terribly gone wrong with those equilibrium angles being 99.1 and 89.8 in Parsley 1.3 We saw a similar thing with phosphonic acid before and you fixed it.
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