Exploring the Ability of Frozen-Density Embedding to Model Induced Circular Dichroism

Abstract

In this study, we present calculations of the circular dichroism (CD) spectra of complexes between achiral and chiral molecules. Nonzero rotational strengths for transitions of the nonchiral molecule are induced by interactions between the two molecules, which cause electronic and/or structural perturbations of the achiral molecule. We investigate if the chiral molecule (environment) can be represented only in terms of its frozen electron density, which is used to generate an effective embedding potential. The accuracy of these calculations is assessed in comparison to full supermolecular calculations. We can show that electronic effects arising from specific interactions between the two subsystems can reliably be modeled by the frozen-density representation of the chiral molecule. This is demonstrated for complexes of 2-benzoylbenzoic acid with (-)-(R)-amphetamine and for a nonchiral, artificial amino acid receptor system consisting of ferrocenecarboxylic acid bound to a crown ether, for which a complex with l-leucine is studied. Especially in the latter case, where multiple binding sites and interactions between receptor and target molecule exist, the frozen-density results compare very well with the full supermolecular calculation. We also study systems in which a cyclodextrin cavity serves as a chiral host system for a small, achiral molecule. Problems arise in that case because of the importance of excitonic couplings with excitations in the host system. The frozen-density embedding cannot describe such couplings but can only capture the direct effect of the host electron density on the electronic structure of the guest. If couplings play a role, frozen-density embedding can at best only partially describe the induced circular dichroism. To illustrate this problem, we finally construct a case in which excitonic coupling effects are much stronger than direct interactions of the subsystem densities. The frozen density embedding is then completely unsuitable.

Equilibrium Geometries of Noncovalently Bound Intermolecular Complexes Derived from Subsystem Formulation of Density Functional Theory

“…The effect of the environment on the electronic structure of the embedded subsystem can be seen as the result of two effects: the environment induced changes of the geometry and the direct electronic effects (for a recent representative analysis, see ref 10). In many cases, the geometry of the investigated system is known from either experiment or computational studies applying other methods.…”

High-accuracy calculation of nuclear quadrupole moments of atomic halogens

“…In particular, for the description of solvent effects, this DFT-in-DFT embedding scheme has been shown to be both an accurate and an efficient method for the calculation of absorption spectra, [3][4][5] electron spin resonance parameters, 6 and nuclear magnetic resonance chemical shifts. 7 It has further been successfully applied to model more complex environments, e.g., for describing induced circular dichroism in host-guest systems 8 or for free-energy calculations in protein environments. 9,10 Recently, Neugebauer has extended the FDE formalism to describe couplings between electronic transitions in different subsystems.…”

Computing the Optical Properties of Large Systems

“…The method of converging localised excitations in different regions A, B ... and then coupling them in a post-processing step by assuming that global transition density matrices can be written as linear combinations of transition density matrices of the individual subsystems is analogous to the coupled frozen-density-embedding TDDFT (FDEc-TDDFT) approach introduced by Neugebauer et al [148][149][150]. However, their approach shows a number of differences, mainly in how the ground state calculation is treated.…”