The Challenge of Accurate Computation of Two-Photon Absorption Properties of Organic Chromophores in the Condensed Phase

Abstract

Two strategies are applied to evaluate the effect of the environment on the two-photon absorption (TPA) cross sections for two characteristic excited states of C 2 H 4 upon complexation with H 2 O. The supermolecular strategy provides the reference complexation-induced shifts and uses either the EOM-CCSD or ADC(2) method. The embedding strategy is based on frozen-density-embedding theory (FDET) and uses only fundamental constants. The TPA cross sections from high-level supermolecular calculations are extremely basis-set-sensitive. Literature data and the present study indicate that accuracy of the absolute TPA cross sections below 100 atomic units and their shifts below 10 atomic units remains a challenge. The obtained FDET results show a similar basis-set behavior. For the largest basis set (d-aug-cc-pVQZ), TPA cross sections obtained from these two strategies are in excellent agreement. The complexation-induced shifts have the correct sign of the effect and a small (12−33%) relative error in magnitude. The deviations of the FDET-derived shifts from the reference are of similar magnitude as the reliability threshold of the reference shifts.

The Seamless Connection of Local and Collective Excited States in Subsystem Time-Dependent Density Functional Theory

“…By going beyond the linear-response regime of sTDDFT, other interesting (multi) photoinduced properties such as two-photon absorption are, in principle, accessible. A first step toward this direction can be found in ref for FDE in combination with ADC(2). Alternatively, higher-order response contributions can be incorporated by means of, for example, real-time sTDDFT. , Recently, the usage of such techniques for finite and extended molecular systems has gained momentum. ,− …”

Excitation Energies of Embedded Chromophores from Frozen-Density Embedding Theory Using State-Specific Electron Densities of the Environment

“…Our previous work showed comprehensively that using ρ B corresponding to the ground-state density of the system comprising only the molecules of the environment (branded Protocol A in the present work), is a good starting point for modeling localized excitations in FDET-based methods. , The average absolute error due to approximations made in FDET and the above choice of ρ B is as small as 0.039 eV in the case of 451 representative excitations considered in ref . Protocol A rarely leads due to approximations made in FDET exceeding 0.1 eV if used for localized excitations in chromophores in noncovalently bound environments.…”

“…The FDET eigenvalue problem is solved only once with a common embedding potential for each and every state. As a result, the eigenfunctions are orthogonal and can be used in a straightforward manner to obtain transition moments or to be used in response-based formalisms . The polarization of the environment (either state-specific or not) is taken into account implicitly if the densities ρ A and ρ B do overlap.…”

“…Finally, the numerical results obtained with a common ρ B yield already rather good vertical excitation energies. Our previous work indicates that using a state-independent environment density ρ B for all electronic states is an adequate choice for excitations localized in the embedded chromophore. , For 351 excitations in 52 clusters, the average absolute error of the excitation energy due to all approximations used in the FDET-based method (including the use of state-independent ρ B ) is as low as 0.039 eV . Neugebauer and collaborators explored the potential benefits of using state-dependent ρ B a method sharing with FDET several elements. − The used method involved several approximations/choices concerning the generation of state-dependent and state-independent ρ B .…”

Ultrafast Evaluation of Two-Photon Absorption with Simplified Time-Dependent Density Functional Theory

“…From a theoretical point of view, computing a higher-order molecular response property such as 2PA cross-sections (σ 2PA ) is inherently difficult . In the realm of wave function methods, probably the most accurate scheme was developed by Nanda and Krylov at the equation-of-motion coupled-cluster (CC) wave functions with single and double substitutions (EOM-CCSD) level of theory, but it is only applicable to relatively small systems. − Other CC methods to evaluate 2PA were implemented by Hättig et al at the CCSD, CCS, and CC2 levels of theory. A CC3 implementation also exists .…”