All-electron real-time and imaginary-time time-dependent density functional theory within a numeric atom-centered basis function framework.

Real-time time-dependent density functional theory (RT-TDDFT) is an attractive tool to model quantum dynamics by real-time propagation without the linear response approximation. Sharing the same technical framework of RT-TDDFT, imaginary-time time-dependent density functional theory (it-TDDFT) is a recently developed robust-convergence ground state method. Presented here are high-precision all-electron RT-TDDFT and it-TDDFT implementations within a numerical atom-centered orbital (NAO) basis function framework in the FHI-aims code. We discuss the theoretical background and technical choices in our implementation. First, RT-TDDFT results are validated against linear-response TDDFT results. Specifically, we analyze the NAO basis sets' convergence for Thiel's test set of small molecules and confirm the importance of the augmentation basis functions for adequate convergence. Adopting a velocity-gauge formalism, we next demonstrate applications for systems with periodic boundary conditions. Taking advantage of the all-electron full-potential implementation, we present applications for core level spectra. For it-TDDFT, we confirm that within the all-electron NAO formalism, it-TDDFT can successfully converge systems that are difficult to converge in the standard self-consistent field method. We finally benchmark our implementation for systems up to ∼500 atoms. The implementation exhibits almost linear weak and strong scaling behavior.

A fresh look at the structure of aromatic thiols on Au surfaces from theory and experiment.

A detailed study of the adsorption structure of self-assembled monolayers of 4-nitrothiophenol on the Au(111) surface was performed from a theoretical perspective via first-principles density functional theory calculations and experimentally by Raman and vibrational sum frequency spectroscopy (vSFS) with an emphasis on the molecular orientation. Simulations-including an explicit van der Waals (vdW) description-for different adsorbate structures, namely, for (3×3), (2 × 2), and (3 × 3) surface unit cells, reveal a significant tilting of the molecules toward the surface with decreasing coverage from 75° down to 32° tilt angle. vSFS suggests a tilt angle of 50°, which agrees well with the one calculated for a structure with a coverage of 0.25. Furthermore, calculated vibrational eigenvectors and spectra allowed us to identify characteristic in-plane (NO2 scissoring) and out-of-plane (C-H wagging) modes and to predict their strength in the spectrum in dependence of the adsorption geometry. We additionally performed calculations for biphenylthiol and terphenylthiol to assess the impact of multiple aromatic rings and found that vdW interactions are significantly increasing with this number, as evidenced by the absorption energy and the molecule adopting a more upright-standing geometry.