It is now well established that conical intersections play an important role in facilitating non-radiative transitions in electronically excited molecules and materials, but modeling nonadiabatic dynamics in their vicinity is extremely challenging.  In the adiabatic representation, singular first- and second-derivative coupling matrix elements arise due to discontinuities in the electronic wave functions at such intersections.  Second-derivative coupling terms are particularly troubling, because they are non-integrable.  Transformation into a quasidiabatic basis eliminates these singularities, but defining a globally well-behaved quasidiabatic basis is a system specific challenge.  We present a new fully quantum mechanical nonadiabatic molecular dynamics ansatz that directly addresses these challenges: diabatized Gaussians on adiabatic surfaces (DGAS).  DGAS employs a basis of vibronic functions that follow the adiabatic potential energy surfaces, but that are continuous at conical intersection points.  This eliminates singularities in the first- and second-derivative coupling matrix elements arising from the Hamiltonian.  Instead, our vibronic basis may change discontinuously in time, leading to singular time-derivative matrix elements.  Fortunately, these new singularities are integrable and can be gracefully handled in practice by interpolating the electronic wave function in time.  The result is a nonadiabatic molecular dynamics ansatz that shares the advantages of the adiabatic and quasidiabatic representations while avoiding their respective pitfalls.

Painless modeling of dynamics near conical intersections

253rd National Meeting (2017)

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