Figure 2.

The $h_{BAH,B{A}_{\chi }}^2$ functional form for sp² acceptors avoids a numeric instability in BA_χ at BAH angle 180°, by smoothly interpolating between in-plane (A) and out-of-plane (B) BAH potentials as a function of BA_χ (C): $h_{BAH,B{A}_{\chi }}^2$. The Lambert-azimuthal projection of $h_{BAH}^1$ (from HBv1) (D), $h_{BAH,B{A}_{\chi }}^2$ (from HBv2) (E) and 3d rendering of E_HBv2 (F) with a linear AHD and contoured at [−1.2,−1.0, and − .78] shows that HBv2 describes two symmetric lobes corresponding to the ideal sp² orbitals, while HBv1 does not.