Charge-Transfer Excitation within a Hybrid-(G)KS Framework through Cartesian Grid DFT

Abstract

Organic molecules that exhibit charge-transfer (CT) excited states are known to play an important role in processes linked to electron transfer properties and molecular conductance. In this article, we present a simple technique based on "Becke's excitation theorem" that offers an accurate picture of these electronic states. It expresses the correlated energy splitting between triplet and its corresponding singlet states by a twoelectron integral, which is numerically evaluated by our recently developed strategy on Cartesian grid. We first examine the consistency of our adopted numerical strategy to evaluate the integral with the original prescribed technique. Then we assess the method on weakly bound CT complexes with three different functionals (BLYP, B3LYP, and LC-BLYP). The accuracy on asymptotic limit of CT excitation is also explored. Finally in order to illustrate the strength and feasibility, it is further extended to a few "challenging" molecules. The method, when employed with hybrid B3LYP functional, turns out to be quite accurate to describe CT excitation energy.

Recent Advances in Cartesian-Grid DFT in Atoms and Molecules

“…The discrete Fourier transforms are incorporated from the FFTW3 package (Frigo and Johnson, 2005). The above features are implemented in the InDFT (Roy et al, 2019) program developed in our laboratory over the years, which has been employed in several practical applications in a series of articles (Roy, 2008a;Roy, 2008b;Roy, 2010a;Roy, 2010b;Roy, 2011;Ghosal et al, 2016;Ghosal et al, 2018;Ghosal et al, 2019;Mandal et al, 2019;Ghosal et al, 2021;Roy et al, 2021;Ghosal and Roy, 2022a;Ghosal and Roy, 2022b;Roy et al, 2022). At this point, this includes some of the frequently used, prominent XC functionals mentioned at relevant places in the article.…”

“…For comparison, literature/experimental results are also provided side by side. Now, energies from restricted open shell or unrestricted calculations are reported as per convergence; we have taken this liberty as the difference between these two calculations is not significant enough to bring any perceptible change in excitation energy (Roy et al, 2022). As we shift from BLYP to LC-BLYP via B3LYP for both PR and TD, a general trend of increment in excitation energy is noticed in both CT complexes.…”

“…NC implies "not converged." More details are available inRoy et al (2022).FIGURE 4 | E 0S for C 2 H 4 − C 2 F 4 versus R. PR 1 , PR 2 ,and PR 3 denote the present result with BLYP, B3LYP, and LC-BLYP functional, and TD 1 , TD 2 , and TD 3 represent the same within TDDFT. Adapted fromRoy et al (2022).…”