Assessment of Density Functional Theory for Describing
the Correlation Effects on the Ground and Excited State Potential
Energy Surfaces of a Retinal Chromophore Model
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Abstract
In
the quest for a cost-effective level of theory able to describe
a large portion of the ground and excited potential energy surfaces
of large chromophores, promising approaches are rooted in various
approximations to the exact density functional theory (DFT). In the
present work, we investigate how generalized Kohn–Sham DFT
(GKS-DFT), time-dependent DFT (TDDFT), and spin-restricted ensemble-DFT
(REKS) methods perform along three important paths characterizing
a model retinal chromophore (the penta-2,4-dieniminium cation) in
a region of near-degeneracy (close to a conical intersection) with
respect to reference high-level multiconfigurational wave function
methods. If GKS-DFT correctly describes the closed-shell charge transfer
state, only TDDFT and REKS approaches give access to the open-shell
diradical, one which sometimes corresponds to the electronic ground
state. It is demonstrated that the main drawback of the usual DFT-based
methods lies in the absence of interactions between the charge transfer
and the diradicaloid configurations. Hence, we test a new computational
scheme based on the State-averaged REKS (SA-REKS) approach, which
explicitly includes these interactions into account. The State-Interaction
SA-REKS (SI-SA-REKS) method significantly improves on the REKS and
the SA-REKS results for the target system. The similarities and differences
between DFT and wave function-based approaches are analyzed according
to (1) the active space dimensions of the wave function-based methods
and (2) the relative electronegativities of the allyl and protonated
Schiff base moieties