Effect of the diradical character on static polarizabilities and two-photon absorption cross sections: A closer look with spin-flip equation-of-motion coupled-cluster singles and doubles method

Abstract

We present static polarizabilities and two-photon absorption (2PA) cross sections for the low-lying electronic states of prototypical diradicals such as benzynes and analogues of m-xylylene and p-quinodimethane computed with the spin-flip equation-of-motion coupled-cluster singles and doubles (EOM-SF-CCSD) method. The static polarizabilities were calculated as analytic second derivatives of the EOM energies, and the 2PA cross sections were calculated using the expectation-value approach. We explain the trends in the nonlinear responses of the SF target states by constructing few-states models based on truncated sum-over-states expressions for these nonlinear properties. By using a Huckel-type treatment of the frontier molecular orbitals that host the unpaired electrons, we rationalize the trends in the dipole interactions between the SF target states relevant in the few-states models. We demonstrate the correlation between the nonlinear responses of these electronic states and the diradical character.

How to stay out of trouble in RIXS calculations within equation-of-motion coupled-cluster damped response theory? Safe hitchhiking in the excitation manifold by means of core–valence separation

“…In contrast to multi-reference approaches, EOM-CC does not involve systemspecific parameterization (e.g., active-space selection), thus satisfying Pople's requirements of theoretical model chemistry 40 that can be used for systematic studies and comparisons between different systems. The EOM-CC framework yields reliable lowerorder properties such as solvatochromic shifts 41 , transition dipole moments 35 , spin-orbit [42][43][44][45] and non-adiabatic couplings [46][47][48] , as well as higher-order properties 49 such as two-photon absorption cross sections [50][51][52][53][54][55] , static and dynamical polarizabilities [56][57][58][59] . Whereas the bulk of prior developments and applications of the EOM-CC methods as well as of the closely related coupled-cluster response theory [60][61][62] were in the VUV regime, these methods are now being extended to the X-ray regime and their performance is being explored for computing, for example, XAS [15][16][17]19,[63][64][65] , XES 24,66 , and RIXS 24,66 spectra.…”

Complex Excited State Polarizabilities in the ADC/ISR Framework

“…Over the last decades, numerous quantum chemical methods have been employed for the calculation of frequency-dependent excited state polarizabilities, including wave function methods such as Hartree-Fock, 10,[14][15][16] coupled-cluster, 11,12,[17][18][19][20][21] and multi-reference approaches, 15,16,18,22 as well as DFT methods. 20,[23][24][25] Calculations of excited state C 6 dispersion coefficients are more scarce 24 and this property is also difficult to determine experimentally.…”

The effect of polarizable environment on two-photon absorption cross sections characterized by the equation-of-motion coupled-cluster singles and doubles method combined with the effective fragment potential approach

“…[31][32][33] Coupled-cluster methods have been employed within the polarizable embedding scheme in the calculations of 2PA cross sections. 22 In this article, we extend our EOM-CCSD implementation of the 2PA cross sections [34][35][36] to include the effect of solvent environment via QM/EFP embedding. We assess the quality of the EOM-EE-CCSD/EFP 2PA cross sections (i.e., computed with the EOM-EE-CCSD wave functions embedded in the polarizable environment of EFP fragments) relative to the full EOM-EE-CCSD calculations and identify the main sources of errors.…”