Abstract
Quantum-chemical computational methods are benchmarked for their ability to describe conical intersections in a series of organic molecules and models of biological chromophores. Reference results for the geometries, relative energies, and branching planes of conical intersections are obtained using ab initio multireference configuration interaction with single and double excitations (MRCISD). They are compared with the results from more approximate methods, namely, the state-interaction state-averaged restricted ensemble-referenced Kohn-Sham method, spin-flip time-dependent density functional theory, and a semiempirical MRCISD approach using an orthogonalization-corrected model. It is demonstrated that these approximate methods reproduce the ab initio reference data very well, with root-mean-square deviations in the optimized geometries of the order of 0.1 Å or less and with reasonable agreement in the computed relative energies. A detailed analysis of the branching plane vectors shows that all currently applied methods yield similar nuclear displacements for escaping the strong non-adiabatic coupling region near the conical intersections. Our comparisons support the use of the tested quantum-chemical methods for modeling the photochemistry of large organic and biological systems.
Highlights
It is widely recognized that conical intersections (CIs) are among the most important mechanistic features in photochemistry and photobiology.1–5 They serve as efficient funnels for non-adiabatic relaxation of electronically excited states and often govern their lifetimes and the branching ratio of photoproducts.4–7 The topography of CIs and the shapes of the corresponding intersecting potential energy surfaces (PESs) are important characteristics that largely determine the non-adiabatic dynamics.7–9 The most important descriptors in this regard are the so-called branching plane vectors,8 i.e., the atomic displacements along which the degeneracy of the electronic states at a CI is lifted
The S0/S1 intersections in these systems, see Figure 3, originate from crossings between the electronic states that correspond to the homolytic and heterolytic breaking of the π -bond.4,22,108–110. These CIs can be classified as twisted-pyramidalized or as twisted-bond_length_alternating depending on the relative preference for the two bond-breaking mechanisms; the tw-pyr CIs are typical of molecules with dominant homolytic bond breaking, while the tw-BLA CIs occur in molecules for which both π -bond breaking processes are nearly isoenergetic
These CIs can be classified as twisted-pyramidalized or as twisted-bond_length_alternating depending on the relative preference for the two bond-breaking mechanisms; the tw-pyr CIs are typical of molecules with dominant homolytic bond breaking, while the tw-BLA CIs occur in molecules for which both π -bond breaking processes are nearly isoenergetic.22,108. Another type of CIs commonly occurring in organic molecules can be vaguely classified as n/π CIs that originate from the crossing between an electron configuration (n2π *0) with a doubly occupied lone pair and a singly excited (n1π *1) configuration
Summary
It is widely recognized that conical intersections (CIs) are among the most important mechanistic features in photochemistry and photobiology. They serve as efficient funnels for non-adiabatic relaxation of electronically excited states and often govern their lifetimes and the branching ratio of photoproducts. The topography of CIs and the shapes of the corresponding intersecting potential energy surfaces (PESs) are important characteristics that largely determine the non-adiabatic dynamics. The most important descriptors in this regard are the so-called branching plane vectors, i.e., the atomic displacements along which the degeneracy of the electronic states at a CI is lifted. It is widely recognized that conical intersections (CIs) are among the most important mechanistic features in photochemistry and photobiology.. It is widely recognized that conical intersections (CIs) are among the most important mechanistic features in photochemistry and photobiology.1–5 They serve as efficient funnels for non-adiabatic relaxation of electronically excited states and often govern their lifetimes and the branching ratio of photoproducts.. An alternative approach based on ensemble DFT is the state-interaction state-averaged restricted ensemble-referenced Kohn-Sham (SI-SA-REKS) method, which has been shown to be suited for describing CIs in organic molecules.. An alternative approach based on ensemble DFT is the state-interaction state-averaged restricted ensemble-referenced Kohn-Sham (SI-SA-REKS) method, which has been shown to be suited for describing CIs in organic molecules.20 Both these DFT-based methods have been applied to optimize CIs and to determine BPs.. In the domain of density functional theory (DFT), the use of the spinflip (SF) ansatz in the context of time-dependent DFT (SFTDDFT) allows the treatment of intersections between ground- and excited-state PESs. An alternative approach based on ensemble DFT is the state-interaction state-averaged restricted ensemble-referenced Kohn-Sham (SI-SA-REKS) method, which has been shown to be suited for describing CIs in organic molecules. Both these DFT-based methods have been applied to optimize CIs and to determine BPs. Semiempirical quantum-chemical methods have become increasingly popular for investigating the non-adiabatic dynamics of excited states, in particular, the OM2/MRCI approach with an orthogonalization-corrected model Hamiltonian (OM2). Taking advantage of its very low computational demands, numerous on-the-fly non-adiabatic molecular dynamics (MD) simulations have been performed with OM2/MRCI for electronically excited organic molecules.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
