Double excitations in finite systems

Double-core excitations in formamide can be probed by X-ray double-quantum-coherence spectroscopy

The attosecond, time-resolved X-ray double-quantum-coherence four-wave mixing signals of formamide at the nitrogen and oxygen K-edges are simulated using restricted excitation window time-dependent density functional theory and the excited core hole approximation. These signals, induced by core exciton coupling, are particularly sensitive to the level of treatment of electron correlation, thus providing direct experimental signatures of electron and core-hole many-body effects and a test of electronic structure theories.

Core and valence excitations in resonant X-ray spectroscopy using restricted excitation window time-dependent density functional theory

We report simulations of X-ray absorption near edge structure (XANES), resonant inelastic X-ray scattering (RIXS) and 1D stimulated X-ray Raman spectroscopy (SXRS) signals of cysteine at the oxygen, nitrogen, and sulfur K and L(2,3) edges. Comparison of the simulated XANES signals with experiment shows that the restricted window time-dependent density functional theory is more accurate and computationally less expensive than the static exchange method. Simulated RIXS and 1D SXRS signals give some insights into the correlation of different excitations in the molecule.

Progress in time-dependent density-functional theory

The classic density-functional theory (DFT) formalism introduced by Hohenberg, Kohn, and Sham in the mid-1960s is based on the idea that the complicated N-electron wave function can be replaced with the mathematically simpler 1-electron charge density in electronic structure calculations of the ground stationary state. As such, ordinary DFT cannot treat time-dependent (TD) problems nor describe excited electronic states. In 1984, Runge and Gross proved a theorem making TD-DFT formally exact. Information about electronic excited states may be obtained from this theory through the linear response (LR) theory formalism. Beginning in the mid-1990s, LR-TD-DFT became increasingly popular for calculating absorption and other spectra of medium- and large-sized molecules. Its ease of use and relatively good accuracy has now brought LR-TD-DFT to the forefront for this type of application. As the number and the diversity of applications of TD-DFT have grown, so too has our understanding of the strengths and weaknesses of the approximate functionals commonly used for TD-DFT. The objective of this article is to continue where a previous review of TD-DFT in Volume 55 of the Annual Review of Physical Chemistry left off and highlight some of the problems and solutions from the point of view of applied physical chemistry. Because doubly-excited states have a particularly important role to play in bond dissociation and formation in both thermal and photochemistry, particular emphasis is placed on the problem of going beyond or around the TD-DFT adiabatic approximation, which limits TD-DFT calculations to nominally singly-excited states.

Efficient moves for global geometry optimization methods and their application to binary systems

We show that molecular dynamics based moves in the minima hopping method are more efficient than saddle point crossing moves. For binary systems we incorporate identity exchange moves in a way that allows one to avoid the generation of high energy configurations. Using this modified minima hopping method, we re-examine the binary Lennard-Jones benchmark system with up to 100 atoms and we find a large number of new putative global minima.

Successes and failures of Kadanoff-Baym dynamics in Hubbard nanoclusters

We study the nonequilibrium dynamics of small, strongly correlated clusters, described by a Hubbard Hamiltonian, by propagating in time the Kadanoff-Baym equations within the Hartree-Fock, second Born, GW, and T-matrix approximations. We compare the results to exact numerical solutions. We find that the time-dependent T matrix is overall superior to the other approximations, and is in good agreement with the exact results in the low-density regime. In the long time limit, the many-body approximations attain an unphysical steady state which we attribute to the implicit inclusion of infinite-order diagrams in a few-body system.

Theoretical study of photochromic compounds. 1. Bond length alternation and absorption spectra for the open and closed forms of 29 diarylethene derivatives

We apply several exchange-correlation functionals in combination with time-dependent density functional theory to predict the maximum wavelengths in the absorption spectra for 29 diarylethene derivatives in both open and closed isomeric forms. Solvent effects and accurate molecular geometries are found to be important to obtain good agreement with experimental absorption wavelengths. In order to evaluate the quality of geometry optimization, we compare predicted bond length alternation parameters with experimental ones. We find the TD-M05/6-31G*/PCM//M05-2x/6-31G*/PCM theory level to give the best predictions for the structural and spectral parameters of the diarylethene derivatives. Applications of the photochromic diarylethene compounds as materials for optical switching and data storage based on their photocyclization properties are also discussed.

Rabi oscillations and few-level approximations in time-dependent density functional theory

The resonant interaction of laser light with atoms is analyzed from the time-dependent density functional theory perspective using a model helium atom which can be solved exactly. It is found that in the exact exchange approximation the time-dependent dipole shows Rabi-type oscillations of its amplitude. However, the time-dependent density itself is not well described. These seemingly contradictory findings are analyzed. The Rabi-type oscillations are found to be essentially of classical origin. The incompatibility of time-dependent density functional theory with few-level approximations for the description of resonant dynamics is discussed.