Analytic gradients, geometry optimization and excited state potential energy surfaces from the particle-particle random phase approximation

Davidson

Proc. Natl. Acad. Sci. U.S.A.

2016

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163

208

Higher acenes have drawn much attention as promising organic semiconductors with versatile electronic properties. However, the nature of their ground state and electronic excited states is still not fully clear. Their unusual chemical reactivity and instability are the main obstacles for experimental studies, and the potentially prominent diradical character, which might require a multireference description in such large systems, hinders theoretical investigations. Here, we provide a detailed answer with the particleparticle random-phase approximation calculation. The 1 A g ground states of acenes up to decacene are on the closed-shell side of the diradical continuum, whereas the ground state of undecacene and dodecacene tilts more to the open-shell side with a growing polyradical character. The ground state of all acenes has covalent nature with respect to both short and long axes. The lowest triplet state 3 B 2u is always above the singlet ground state even though the energy gap could be vanishingly small in the polyacene limit. The bright singlet excited state 1 B 2u is a zwitterionic state to the short axis. The excited 1 A g state gradually switches from a doubleexcitation state to another zwitterionic state to the short axis, but always keeps its covalent nature to the long axis. An energy crossing between the 1 B 2u and excited 1 A g states happens between hexacene and heptacene. Further energetic consideration suggests that higher acenes are likely to undergo singlet fission with a low photovoltaic efficiency; however, the efficiency might be improved if a singlet fission into multiple triplets could be achieved.higher acenes | diradical | double excitation | particle-particle random-phase approximation | charge-transfer excitation

Section: Significancementioning

confidence: 99%

Nature of ground and electronic excited states of higher acenes

Davidson

Proc. Natl. Acad. Sci. U.S.A.

2016

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163

208

“…25,29 Note that the noncanonical form of the generalized ph- or pp-RPA eigenvalue problem is used. 47 Because this expression is invariant with rotations, as we have shown (see the SI), we can perform the optimization in the occupied-virtual space only. The ph-RPA can be derived within the framework of DFT via the adiabatic-connection fluctuation—dissipation (ACFD) theorem; 23,48–50 the pp-RPA can also be derived in the equivalence of ACFD in the pairing channel.…”

mentioning

confidence: 99%

Generalized Optimized Effective Potential for Orbital Functionals and Self-Consistent Calculation of Random Phase Approximations

Jin

Zhang

Chen

et al. 2017

J. Phys. Chem. Lett.

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A new self-consistent procedure for calculating the total energy with an orbital-dependent density functional approximation (DFA), the generalized optimized effective potential (GOEP), is developed in the present work. The GOEP is a nonlocal Hermitian potential that delivers the sets of occupied and virtual orbitals and minimizes the total energy. The GOEP optimization leads to the same minimum as does the orbital optimization. The GOEP method is promising as an effective optimization approach for orbital-dependent functionals, as demonstrated for the self-consistent calculations of the random phase approximation (RPA) to the correlation functionals in the particle—hole (ph) and particle—particle (pp) channels. The results show that the accuracy in describing the weakly interacting van der Waals systems is significantly improved in the self-consistent calculations. In particular, the important single excitations contribution in non-self-consistent RPA calculations can be captured self-consistently through the GOEP optimization, leading to orbital renormalization, without using the single excitations in the energy functional.

“…The pp-RPA and pp-TDA were also further applied to excited states calculations including both excitation energy calculation 17 and geometry optimization. 20 These pp methods start from a two-electron defficient (N-2) reference, and target the neutral (N) states by adding two electrons, thus describing the ground and excited states on the same footing. Similar to SF-TDDFT, the interaction between ground and excited states are naturally taken into consideration.…”

mentioning

confidence: 99%

Conical Intersections from Particle–Particle Random Phase and Tamm–Dancoff Approximations

Shen

Zhang

et al. 2016

J. Phys. Chem. Lett.

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The particle-particle random phase approximation (pp-RPA) and the particle-particle Tamm-Dancoff approximation (pp-TDA) are applied to the challenging conical intersection problem. Since they describe the ground and excited states on the same footing and naturally take into account the interstate interaction, these particle-particle methods, especially the pp-TDA, can correctly predict the dimensionality of the conical intersection seam as well as describe the potential energy surface in the vicinity of conical intersections. Though the bond length of conical intersections is slightly underestimated compared with the complete-active-space self consistent field (CASSCF) theory, the efficient particle-particle methods are promising for conical intersections and non-adiabatic dynamics.