Electronic Structure

A full coupled-cluster (FCC) expansion for arbitrary electronic systems is developed by explicitly expanding the commutators of the Baker-Campbell-Hausdorff series for cluster operators in binary representations. A full coupled-cluster reduction (FCCR) [1], that is capable of providing very accurate solutions of the many-body Schrödinger equation exploiting the sparsity of the exponential ansatz of FCC,is then introduced employing screenings to the projection manifold and commutator operations. The projection manifold is iteratively updated using the interaction space connected to the primary clusters with large amplitudes. The operation of the commutators is further reduced by introducing a correction, taking into account the so-called exclusion-principle-violating (EPV) terms that provides a fast and near-variational convergence in many cases. Numerical results will be presented for systems including polyacenes and the chromium dimer. We shall further discuss a partially linearized adaptation of FCCR(l) [2], which allows a massively parallel implementation for a large excitation manifold.

[1] E. Xu, M. Uejima, and S. L. Ten-no, Phys. Rev. Lett., 121113001 (2018).
[2] E. Xu, M. Uejima, and S. L. Ten-no, in preparation.

Any effective Hamiltonian based many-body formalism describing a manifold of states dominated by a set of ‘model functions’ spanning a CAS is numerically unstable due to the notorious problem of intruder states. This is why generating PES of a group of states can rarely be studied in a size-consistent manner via an H_eff. CAS based state-specific theories which target just one root of interest at a time has the potentiality of bypassing intruders, yet providing accurate PES. The state-specific multi-reference coupled cluster and perturbation theory (SS-MRCC/PT) of Mukherjee et al realized this goal and it has been very successful in describing multiple bond-breaking in a size-consistent manner. For studying states of different spin-multiplicities, we require a spin free theory to avoid spin contamination and the attendant spuriosities. We have used our Unitary Group Adapted SSMRPT (UGA-SSMRPT), which possesses all the desirable features of UGASSMRCC but computationally much cheaper, for generating a manifold of PES of same or different spatial symmetries. One expects in such situations interlacing for the PES of different symmetry and strong/weak avoided crossings for states of same symmetry. It is not immediately clear how a state-specific theory, generating successively higher-lying PES one at a time, would retain sufficiently accurate information of the lower lying wavefunctions to demonstrate the above features. We present here the results for a variety of electronic states of a set of diatomics in their various spin multiplicities which display striking accuracy by the rather low order UGA-SSMRPT for all the states studied by us. To get the best results dynamic state averaging at the CASSCF level appears to be very important. Accuracy of our results has been benchmarked against IC-MRCISD+Q. We will discuss the implications of such an accomplishment in some detail.

Fock space coupled-cluster is a well-established method to study ionization, electron affinity and excited states in a direct manner using 1-hole, 1-particle and 1 hole-particle model space respectively at singles and doubles level (FSCCSD) ¹. The method is based on diagonalization of an effective Hamiltonian constructed within the sectors of holes and particles to which model space belongs. The normal ordering of wave operator used in Fock space leads to partial decoupling and the higher Fock space solutions require progressive solutions at lower sectors. Thus, for excited states using 1 hole-particle model space, solutions to 1 hole and 1 particle are also obtained, leading to direct ionization and electron attachment energies. It was realized that the inclusion of triples is necessary to improve the quality of FSCC description of direct calculation of all these quantities. However, the full description of triples (FSCCSDT) is unfeasible in terms of computer time and storage capacities. Therefore non-iterative perturbation approximation is designed as a compromise between the cost and performance of full model in improvement of wave function and energy due to the inclusion of triples. An improvement to ionization was done by inclusion of triples at third and upto fourth order on top of full FSCCD calculation ². In this presentation, inclusion of all triples, up to fourth order for 1-particle Fock space sector as well as 1hole-1 particle Fock space, is presented.  This will lead to improved calculateion of electron affinity and excitation energy.

According to when the static and dynamic components of electron correlation are treated, the available wave function-based correlation methods can be classified into three families, viz., "static-then-dynamic", "dynamic-then-static", and "static-dynamic-static (SDS)" [1]. Herewith we report a restricted SDS framework [2], which employs the same number (Np; the number of target states) of primary, secondary and external states for describing the static, dynamic, and again static components of correlation. That is, the secular equation to be diagonalized is of dimension 3Np, irrespective of the numbers of correlated electrons and orbitals. Even the lowest-order realizations of this seemingly restricted SDS framework, i.e., SDSPT2 and SDSCI, are already very accurate for classic test problems of variable degeneracies [2,3], whereas a high-order realization, i.e., iCI (iterative Configuration Interaction), can converge monotonically and quickly to full CI from above, even when a rather poor reference is taken as the start [1]. Interestingly, the micro-iteration of iCI can be reformulated as an iterative Vector Interaction (iVI) method for exterior or interior roots of general large matrices[4,5]. In this lecture, we will introduce a tabulated orbital-configuration based unitary group approach (TOC-UGA) for the selection of important configurations in iCI, so as to make the latter as efficient as possible. 

1. W. Liu and M. R. Hoffmann, J. Chem. Theory Comput. 2016, 12, 1169; (E) 2016, 12, 3000. 

2. W. Liu and M. R. Hoffmann, Theor. Chem. Acc. 2014, 133, 1481.

3. Y. Lei, W. Liu, and M. R. Hoffmann, Mol. Phys. 2017, 115, 2696.

4. C. Huang, W. Liu, Y. Xiao, and M. R. Hoffmann, J. Comput. Chem. 2017, 38, 2481; (E) 2018, 39, 338.

5. C. Huang and W. Liu, J. Comput. Chem. 2019, 40, 1023.

Organic light-emitting diodes (OLEDs) are widely viewed as the basis for next generation displays and lighting. However, to apply this technology widely and for it to reach its full potential, improvements in the emission efficiency and device lifetime are vital. So far, the development of blue, and especially deep blue, emitters in OLEDs has progressed rather slowly. Therefore being able to predict the emission efficiency prior to the synthesis and measurement of properties is of crucial importance for the discovery of new highly efficient blue emitters. Our recent work^1,2 has shown that it is possible to predict the emission efficiency through the calculation of the radiative rate³ and non-radiative rate^1,2 using state-of-the-art computational strategies. Specifically, we find that the main non-radiative process of a series of iridium(III) complexes is the elongation or even breaking of a metal-ligand bond, and the non-radiative rate can be predicted from the energy barrier to this non-emissive state. In this talk, I will present our recent advances in predicting the emission efficiency of a series of deep blue light-emitting complexes that allows us further proposing new highly efficient deep blue emitters for OLEDs.⁴

References:

1. Zhou X., Burn P. L., and Powell B. J. Inorg. Chem. 2016, 55, 5266–5273.

2. Zhou X., Burn P. L., and Powell B. J. J. Chem. Phys. 2017, 146, 174305.

3. Powell, B. J. Coord. Chem. Rev. 2015, 295, 46–79.

4. Zhou X. and Powell B. J. Inorg. Chem. 2018, 57, 8881-8889.

Organic light emitting diodes (OLEDs) represent a disruptive technology that has changed the display industry. A key component of significantly increasing the efficiency of OLED devices is employing transition metal-based triplet emitters that theoretically are capable of showing quantitative phosphorescence efficiencies. We report a new formulation for Golden Rule-based predictions of photoluminescence quantum yields (PLQY) of phosphorescent emitters containing a heavy element, and its implementation compatible with first-principles computation frameworks. We applied this new approach to the photophysical properties of 34 Pt(II) complexes designed for the organic light-emitting diode (OLED) applications and observed a good agreement between predictions and experiments over diverse scaffolds.

Although we have figured out that intrinsic degradation of the materials composing emitting layers is responsible for the short operation lifetime, the mechanism underlying the degradation processes have yet to be fully understood. We examined how to enhance the lifetime of organic light-emitting diodes (OLEDs) based on bipolar host molecules ET-HT, where ET and HT refer to electron- and hole-transporting units, respectively, by analyzing their thermodynamic and kinetic stabilities. Our DFT calculations reveal that the thermodynamic stability of ET-HT is determined by that of its anion, which is difficult to improve by chemical modifications of ET and HT. The kinetic stability of ET-HT can be enhanced by the spiroconjugation between ET and We examined how to enhance the lifetime of organic light-emitting diodes (OLEDs) based on bipolar host molecules Green OLED devices were fabricated by using ET-HTs with and without spiroconjugation, to find that the device with spiroconjugation has a lifetime that is approximately 6 times longer than the one without spiroconjugation.