It is widely understood that the theoretical methods used for studying electronic excited states are extremely limiting. Presently, theoretical methods for describing excited states can broadly be divided into two categories- single-reference response methods and multireference methods. Single reference methods are reliable for molecules with single excitations to states that can be described by a single determinate. However, these methods cannot describe doubly excited states or states that involve multiple determinants especially around the conical intersection between the ground and first excited state. Multireference methods are alternatives for studying excited states and are relatively accurate if used correctly. However, these methods are known for their difficulty and can become extremely computationally costly based on the size of the system used. A third type of methodology, ΔSCF has become of interest due to the orbital relaxation built into the methodology but it lacks the balanced treatment of both static and dynamic correlation.<br/><br/>In this work, we discuss the usage of projection-after-variation double-hybrid density functional theory methodology for examining excited states. In which, ΔSCF methodology is used with double-hybrid density functional methodology to recover a more balanced result of correlation. The strengths and weaknesses of the proposed method are discussed with particular reference to connections with linear response coupled-cluster theory. Vertical excitation energies are computed for the 28 molecule benchmark of Schreiber and co-workers in order to compare how the model performs with linear response coupled-cluster theories and multireference perturbation theory. The findings of this study show that the proposed method can achieve standard deviations in the error of computed vertical excitation energies compared to complete active space second-order perturbation theory of similar size to linear response coupled-cluster theories.

Difference projection-after-variation double-hybrid density functional theory applied to the calculation of vertical excitation energies

ACS Spring Meeting 2021

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