Charge transfer in time-dependent density functional theory

Abstract

Charge transfer plays a crucial role in many processes of interest in physics, chemistry, and biochemistry. In many applications the size of the systems involved calls for time-dependent density functional theory (TDDFT) to be used in their computational modeling, due to its unprecedented balance between accuracy and efficiency. However, although exact in principle, in practise approximations must be made for the exchange-correlation functional in this theory, and the standard functional approximations perform poorly for excitations which have a long-range charge-transfer component. Intense progress has been made in developing more sophisticated functionals for this problem, which we review. We point out an essential difference between the properties of the exchange-correlation kernel needed for an accurate description of charge-transfer between open-shell fragments and between closed-shell fragments. We then turn to charge-transfer dynamics, which, in contrast to the excitation problem, is a highly non-equilibrium, non-perturbative, process involving a transfer of one full electron in space. This turns out to be a much more challenging problem for TDDFT functionals. We describe dynamical step and peak features in the exact functional evolving over time, that are missing in the functionals currently used. The latter underestimate the amount of charge transferred and manifest a spurious shift in the charge transfer resonance position. We discuss some explicit examples.

Modeling absorption spectra of molecules in solution

“…[59,70,92,93] Conventional local and semi-local functionals do not have the correct long-range spatial dependence to account for the Coulombic interaction between an excited electron and hole, and as a result the charge-transfer excitation energies are usually very close to the occupied-virtual orbital energy difference. [94][95][96] The short-range solute-solvent interactions can be modeled by extracting the closest solvent molecules around a solute from a larger-scale MD simulation. If the long-range electrostatic screening is neglected during this process, the semi-local DFT description of the orbitals of the solvent molecules often predicts an overly small band gap, [70,71] leading to many low-energy charge transfer transitions between the solvent and solute.…”

Double hybrids and time‐dependent density functional theory: An implementation and benchmark on charge transfer excited states

“…Since the results obtained are extremely encouraging in term of accuracy, in the present article, we present the implementation and benchmarking of TD(A)‐DFT approaches focusing on the analysis of their performance for the description of through space charge‐transfer (CT) excitations which are well known to be very problematic to be described using standard global hybrid approaches …”

“…[18,23] Since the results obtained are extremely encouraging in term of accuracy, in the present article, we present the implementation and benchmarking of TD(A)-DFT approaches focusing on the analysis of their performance for the description of through space chargetransfer (CT) excitations which are well known to be very problematic to be described using standard global hybrid approaches. [24][25][26][27][28][29] Two different families of functionals will be compared: those derived imposing the fulfillment of some theoretical constraints, and those based on a more empirical fitting approach. For each of these families the performance of pure (GGA), hybrid (global and range separated hybrids) or DH functionals will be firstly assessed on standard benchmark and next analyzed using a set of charge transfer dimers, recently investigated by Baer and collaborators, as test case.…”

Time‐dependent simulation of ion stopping: Charge transfer and electronic excitations

“…However, there are open questions concerning, for example, the resonant charge transfer in particular at low kinetic energies and the effect of the approximation to the XC-potential. [37,38] Moreover, technological advances allow for the treatment of more extended systems. Here we present a TDDFT-MD simulation of the charge transfer and energy dissipation for an H + ion with initial kinetic energy 2 eV ... 50 eV incident on an Al(111) metal surface (modelled by a cluster), and compare to the energy dissipation in case of an incident H-atom.…”