Impact of Diabolical Singular Points on Nonadiabatic Dynamics and a Remedy: Photodissociation of Ammonia in the First Band.

Several recent publications have pointed out a potentially severe drawback in some widely used diabatization methods based on the electronic properties of molecules. In a diabatic representation defined by a property-based method, artificial singularities may arise due to the defining equation of the adiabatic-to-diabatic (AtD) transformation. Such diabolical singular points (DSPs) may seriously affect nuclear dynamics if they lie in the relevant configuration space. Their impact is demonstrated here using the A-band photodissociation of ammonia as an example. To this end, quantum dynamics calculations are performed based on a diabatic potential energy matrix (DPEM) constructed using the generalized Mulliken-Hush method, which is based on dipoles. These property-based results are compared with the results obtained with a DPEM determined using derivative coupling explicitly. A DSP seam is found near the Franck-Condon region, which results in a complete failure to reproduce the absorption spectrum. A modification of the generalized Mulliken-Hush method is proposed to remove the DSPs while preserving the conical intersection, which leads to an accurate reproduction of the absorption spectrum and the NH2(Ã)/NH2(X̃) product branching ratio.