Equivalence of particle-particle random phase approximation correlation energy and ladder-coupled-cluster doubles

Davidson

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

2016

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169

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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169

208

“…Pp-RPA is the rst density-functional approximation (DFA) to satisfy the at-plane condition exactly 11,12 , and outperforms traditional direct particle-hole random phase approximation in many aspects 13 . Theoretical analysis reveals that the correlation energy from pp-RPA with Hartree-Fock references is equivalent to ladder-coupled-cluster doubles 14,15 . Additionally, the N ± 2 excitation energies from pp-RPA can be used to capture valence, double, charge transfer, and Rydberg excitations with a potential O(N 4 ) scaling 16 .…”

Section: Introductionmentioning

confidence: 99%

Linear-response time-dependent density-functional theory with pairing fields

Peng

Aggelen

The Journal of Chemical Physics

et al. 2014

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Recent development in particle-particle random phase approximation (pp-RPA) broadens the perspective on ground state correlation energies (van Aggelen et al. Phys. Rev. A, 88, 030501 (2013)) and N ± 2 excitation energies (Yang, et al. J.Chem. Phys.). So far Hartree-Fock and approximated density-functional orbitals have been utilized to evaluate the pp-RPA equation. In this paper, to further explore the fundamentals and the potential use of pairing matrix dependent functionals, we present the linear-response time-dependent density-functional theory with pairing elds with both adiabatic and frequency-dependent kernels. This theory is related to the density-functional theory and time-dependent density-functional theory for superconductors, but is applied to normal non-superconducting systems for our purpose. Due to the lack of the proof of the one-to-one mapping between the pairing matrix and the pairing eld for time-dependent systems, the linear-response theory is established based on the representability assumption of the pairing matrix. The linear response theory justi es the use of approximated density-functionals in the pp-RPA equation. This work sets the fundamentals for future density-functional development to enhance the description of ground state correlation energies and N ± 2 excitation energies.

The Journal of Chemical Physics

“…Within KS-DFT, fully nonlocal XC functionals, such as those based on the random phase approximation (RPA), may be adopted for a reliably accurate description of strong static correlation effects. However, RPA-type functionals remain computationally very demanding for large systems [4,13,43,44].…”

Section: Introductionmentioning

confidence: 99%

Role of exact exchange in thermally-assisted-occupation density functional theory: A proposal of new hybrid schemes

Chai

2017

138

We propose hybrid schemes incorporating exact exchange into thermally-assisted-occupation density functional theory (TAO-DFT) [J.-D. Chai, J. Chem. Phys. 136, 154104 (2012)] for an improved description of nonlocal exchange effects. With a few simple modifications, global and range-separated hybrid functionals in Kohn-Sham density functional theory (KS-DFT) can be combined seamlessly with TAO-DFT. In comparison with global hybrid functionals in KS-DFT, the resulting global hybrid functionals in TAO-DFT yield promising performance for systems with strong static correlation effects (e.g., the dissociation of H 2 and N 2 , twisted ethylene, and electronic properties of linear acenes), while maintaining similar performance for systems without strong static correlation effects. Besides, a reasonably accurate description of noncovalent interactions can be efficiently achieved through the inclusion of dispersion corrections in hybrid TAO-DFT. Relative to semilocal density functionals in TAO-DFT, global hybrid functionals in TAO-DFT are generally superior in performance for a wide range of applications, such as thermochemistry, kinetics, reaction energies, and optimized geometries.