KineticNet: Deep learning a transferable kinetic energy functional for orbital-free density functional theory

Orbital-free density functional theory (OF-DFT) holds the promise to compute ground state molecular properties at minimal cost. However, it has been held back by our inability to compute the kinetic energy as a functional of the electron density only. We here set out to learn the kinetic energy functional from ground truth provided by the more expensive Kohn-Sham density functional theory. Such learning is confronted with two key challenges: Giving the model sufficient expressivity and spatial context while limiting the memory footprint to afford computations on a GPU; and creating a sufficiently broad distribution of training data to enable iterative density optimization even when starting from a poor initial guess. In response, we introduce KineticNet, an equivariant deep neural network architecture based on point convolutions adapted to the prediction of quantities on molecular quadrature grids. Important contributions include convolution filters with sufficient spatial resolution in the vicinity of the nuclear cusp, an atom-centric sparse but expressive architecture that relays information across multiple bond lengths; and a new strategy to generate varied training data by finding ground state densities in the face of perturbations by a random external potential. KineticNet achieves, for the first time, chemical accuracy of the learned functionals across input densities and geometries of tiny molecules. For two electron systems, we additionally demonstrate OF-DFT density optimization with chemical accuracy.Comment: 10 pages, 8 figure

Tuning UV Pump X‑ray Probe Spectroscopy on the Nitrogen K Edge Reveals the Radiationless Relaxation of Pyrazine: <i>Ab Initio</i> Simulations Using the Quasiclassical Doorway–Window Approximation

Transient absorption UV pump X-ray probe spectroscopy has been established as a versatile technique for the exploration of ultrafast photoinduced dynamics in valence-excited states. In this work, an ab initio theoretical framework for the simulation of time-resolved UV pump X-ray probe spectra is presented. The method is based on the description of the radiation–matter interaction in the classical doorway–window approximation and a surface-hopping algorithm for the nonadiabatic nuclear excited-state dynamics. Using the second-order algebraic–diagrammatic construction scheme for excited states, UV pump X-ray probe signals were simulated for the carbon and nitrogen K edges of pyrazine, assuming a duration of 5 fs of the UV pump and X-ray probe pulses. It is predicted that spectra measured at the nitrogen K edge carry much richer information about the ultrafast nonadiabatic dynamics in the valence-excited states of pyrazine than those measured at the carbon K edge

Influence of Core Substitution on the Electronic Structure of Benzobisthiadiazoles

Benzobisthiadiazoles (BBTs) are promising organic semiconductors for applications in field effect transistors and solar cells since they possess a strong electron-accepting characteristic. Thereby, the electronic structure of organic/metal interfaces and within thin films is essential for the performance of organic electronic devices. Here, we study the structural and electronic properties of two BBTs, with different core substitution patterns, a phenyl (BBT-Ph) and a thiophene (BBT-Th) derivative adsorbed on Au(111) using vibrational and electronic high-resolution electron energy loss spectroscopy in combination with state-of-the-art quantum chemical calculations. In the mono- and multilayer, both BBTs adopt a planar adsorption geometry with the molecular backbone, as well as the phenyl and thiophene side groups are oriented parallel to the gold substrate. The energies of the lowest excited electronic singlet states (S) and the first triplet state (T1) are determined. The optical gap (S0 → S1 transition) is found to be 2.2 eV for BBT-Ph and 1.6 eV for BBT-Th. The energy of T1 is identified to be 1.2 eV in BBT-Ph and in the case of BBT-Th 0.7 eV. Thus, both the optical gap size as well as the T1 energy are drastically reduced in BBT-Th compared to BBT-Ph. Based on our quantum chemical calculations, this is attributed to the electron-rich nature of the five-membered thiophene rings in conjunction with their preference for planar geometries. Variation of the substitution pattern in BBTs opens an opportunity for tailoring their electronic properties