An electrostatic interaction model for frequency-dependent polarizability: methodology and applications to hydrocarbons and fullerenes

Minimal Auxiliary Basis Set Approach for the Electronic Excitation Spectra of Organic Molecules

We report a minimal auxiliary basis model for time-dependent density functional theory (TDDFT) with hybrid density functionals that can accurately reproduce excitation energies and absorption spectra from TDDFT while reducing cost by about 2 orders of magnitude. Our method, dubbed TDDFT-ris, employs the resolution-of-the-identity technique with just one s-type auxiliary basis function per atom for the linear response operator, where the Gaussian exponents are parametrized across the periodic table using tabulated atomic radii with a single global scaling factor. By tuning on a small test set, we determine a single functional-independent scale factor that balances errors in excitation energies and absorption spectra. Benchmarked on organic molecules and compared to standard TDDFT, TDDFT-ris has an average energy error of only 0.06 eV and yields absorption spectra in close agreement with TDDFT. Thus, TDDFT-ris enables simulation of realistic absorption spectra in large molecules that would be inaccessible from standard TDDFT.

A new framework for frequency-dependent polarizable force fields

A frequency-dependent extension of the polarizable force field "Atom-Condensed Kohn-Sham density functional theory approximated to the second-order" (ACKS2) [Verstraelen et al., J. Chem. Phys. 141, 194114 (2014)] is proposed, referred to as ACKS2ω. The method enables theoretical predictions of dynamical response properties of finite systems after partitioning of the frequency-dependent molecular response function. Parameters in this model are computed simply as expectation values of an electronic wavefunction, and the hardness matrix is entirely reused from ACKS2 as an adiabatic approximation is used. A numerical validation shows that accurate models can already be obtained with atomic monopoles and dipoles. Absorption spectra of 42 organic and inorganic molecular monomers are evaluated using ACKS2ω, and our results agree well with the time-dependent DFT calculations. Also for the calculation of C₆ dispersion coefficients, ACKS2ω closely reproduces its TDDFT reference. When parameters for ACKS2ω are derived from a PBE/aug-cc-pVDZ ground state, it reproduces experimental values for 903 organic and inorganic intermolecular pairs with an MAPE of 3.84%. Our results confirm that ACKS2ω offers a solid connection between the quantum-mechanical description of frequency-dependent response and computationally efficient force-field models.

Nonequilibrium Environment Dynamics in a Frequency-Dependent Polarizable Embedding Model

Hybrid quantum mechanical/molecular mechanical (QM/MM) models are some of the most powerful and computationally feasible approaches to account for solvent effects or more general environmental perturbations on quantum chemical systems. In their more recent formulations (known as polarizable embedding) they can account for electrostatic and mutual polarization effects between the QM and the MM subsystems. In this paper, a polarizable embedding scheme based on induced dipoles that is able both to describe electron evolution of the embedded QM system in an efficient manner as well as to capture the frequency dependent behavior of the solvent is proposed, namely, ωMMPol. The effects of this frequency-dependent solvent on a time-dependent model system-the Rabi oscillations of H₂⁺ in a resonant field-are considered. The solvent is shown to introduce only mild perturbations when the excitation frequencies of the solvent and the solute are off-resonant. However, the dynamics of the H₂⁺ are fundamentally changed in the presence of a near-resonant excitation solvent. The effectiveness of ωMMPol to simulating realistic chemical systems is demonstrated by capturing charge transfer dynamics within a solvated system.

Local electric field factors by a combined charge-transfer and point–dipole interaction model

A model for the local electric field as a linear response to a frequency-dependent external electric field is presented based on a combined charge-transfer and point–dipole interaction force-field model.

Electromagnetic response of polymer composites with quasi-spherical nanocarbon inclusions: theory below the percolation threshold

The linear electromagnetic response of polymer composite with nanocarbon inclusions was modeled for a random multi-component system with particles and clusters of different shape and size. The results of the generalized effective medium Maxwell-Garnett theory below the percolation threshold demonstrate good coincidence with the experimental data for onion-like carbon-based polyurethane composites collected in a quasi-static regime (at 129 Hz) in a broad temperature range.

Enhanced polarizability of aromatic molecules placed in the vicinity of silver clusters

We use a charge-dipole interaction model to study the polarizability of aromatic molecules that are placed between two silver clusters. In particular we examine the enhancement in polarizability induced by the clusters at plasmon-like resonant frequencies of the cluster-molecule-cluster system. The model used for these simulations relies on representation of the atoms by both a net electric charge and a dipole. By relating the time variation of the atomic charges to the currents that flow through the bonds of the structures considered, a least-action principle can be formulated that enables the atomic charges and dipoles to be determined. We consider benzene, naphthalene and anthracene for this study, comparing the polarizability of these aromatic molecules when placed in the middle between two Ag(120) clusters, with their polarizability as isolated molecules. We find that the polarizability of these molecules is enhanced by the clusters, and this increases the electromagnetic coupling between the two clusters. This results in significant red-shifting (by up to 0.8 eV) of the lowest energy optical transition in the cluster-molecule-cluster system compared to plasmon-like excitation in the cluster-cluster system. The resulting resonant polarizability enhancement leads to an electromagnetic enhancement in surface-enhanced Raman scattering of over 10(6).

A charge-dipole interaction model for the frequency-dependent polarizability of silver clusters

We present a charge-dipole interaction model for the calculation of the frequency-dependent polarizability of silver clusters. The model relies on the representation of silver atoms by both a net electric charge and a dipole. Time variations of the atomic charges are related to the currents that flow through the bonds of the structures considered and the atomic charges and dipoles are eventually determined from the application of a least-action principle. After a generalization that enables the bonds of the bulk and surface atoms to have specific resistances, the model is parameterized on data obtained by the time-dependent density functional theory for tetrahedral Ag(20), Ag(84) and Ag(120) clusters. We then study the polarization properties of dimers of silver clusters. We compare in particular the polarizability of the dimers with that of the isolated clusters, for a range of gap distances and frequencies. We also consider the field enhancements one can achieve with these systems. The results are in good agreement with reference data and enable an extension of these data to a wider range of situations. They show that significant field enhancements are achieved at frequencies associated with resonant polarization along the axis of the dimer.