Minimal-active-space multistate density functional theory for excitation energy involving local and charge transfer states

Christian P. Hettich,Xin Chen,Ruoqi Zhao,Jiali Gao

doi:10.1038/s41524-021-00624-3

Abstract

Multistate density functional theory (MSDFT) employing a minimum active space (MAS) is presented to determine charge transfer (CT) and local excited states of bimolecular complexes. MSDFT is a hybrid wave function theory (WFT) and density functional theory, in which dynamic correlation is first incorporated in individual determinant configurations using a Kohn–Sham exchange-correlation functional. Then, nonorthogonal configuration-state interaction is performed to treat static correlation. Because molecular orbitals are optimized separately for each determinant by including Kohn–Sham dynamic correlation, a minimal number of configurations in the active space, essential to representing low-lying excited and CT states of interest, is sufficient to yield the adiabatic states. We found that the present MAS-MSDFT method provides a good description of covalent and CT excited states in comparison with experiments and high-level computational results. Because of the simplicity and interpretive capability through diabatic configuration weights, the method may be useful in dynamic simulations of CT and nonadiabatic processes.

Highlights

Charge transfer (CT) states are electronically excited states which are of fundamental importance in materials and chemical processes, including those in photosynthesis, photoreceptor proteins, catalysis, and photovoltaic devices
(1) CT resonance states Recently, we investigated the significance of CT stabilization of where EIMS1⁄2ρI is the energy of adiabatic state I, HA1⁄2ρA is the Kohn–Sham DFT energy for block-localized determinant A in the active space, corresponding to a diagonal matrix element of the nonorthogonal state interaction (NOSI) Hamiltonian, HATDBF1⁄2ρAB; ΨA; ΨB is a transition density functional (TDF) of the off-diagonal matrix element, defining electronic coupling between states A and B, and the coefficients {c} are obtained by solving the generalized secular equation HC = SCE
State to reproduce the energy of a contracted state of the full configuration interaction (FCI) wave function through singular value decomposition (SVD)[27]

Summary

Introduction

Charge transfer (CT) states are electronically excited states which are of fundamental importance in materials and chemical processes, including those in photosynthesis, photoreceptor proteins, catalysis, and photovoltaic devices. High-level quantum-chemical methods can be used; in practice, it is often difficult to find an approach that can satisfy both requirements simultaneously. Most computational models for excited states are based on multiconfiguration self-consistent-field (MCSCF) approaches such as the complete-active-space self-consistent-field (CASSCF) method. Even at the simplest level of single-excitation configuration interaction (CIS) and linear-response time-dependent density functional theory (TDDFT), which is known to perform poorly on CT states, the large number of configurations included in these calculations makes the interpretation of CT states difficult[2,3,4,5]. We present an approach that employs a minimal active space (MAS) in multistate density functional theory (MSDFT) to treat CT states as well as local valence excitations on a range of bimolecular complexes. The present method provides a straightforward interpretation of results

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Results