Several technologically important processes such as solar energy capture in organic photovoltaics and photocatalysis prominently feature electronically non-adiabatic behavior. Tully’s surface hopping with electronic potential energy surfaces provided by on-the-fly time-dependent density functional theory (TDDFT) calculations is an increasingly powerful method for investigating photochemical reaction mechanisms from first principles. Recent implementations of non-adiabatic coupling calculations between TDDFT excited states base on quadratic response have revealed that transition properties between two excited states become unphysical whenever the energy difference between the two states of interest matches any other excitation energy of the reference system. I will trace this unphysical behavior to spurious poles in the quadratic response function itself. I will further show that this incorrect pole structure is not limited to TDDFT, but in fact is present in the quadratic response functions of many approximate electronic structure methods, including coupled cluster (CC) and multi-configuration self consistent field (MCSCF). These extra poles all have a common origin which will be diagnosed. Finally, I will close by discussing the ramifications for response-based non-adiabatic molecular dynamics and non-linear spectroscopies such as two-photon absorption amplitudes.

Diagnosis and implications of spurious poles in the quadratic response of approximate, electronic structure method

251st National Meeting (2016)

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