Assessment of Electron Propagator Methods for the Simulation of Vibrationally Resolved Valence and Core Photoionization Spectra

Abstract

The analysis of photoelectron spectra is usually facilitated by quantum mechanical simulations. Because of the recent improvement of experimental techniques, the resolution of experimental spectra is rapidly increasing, and the inclusion of vibrational effects is usually mandatory to obtain a reliable reproduction of the spectra. With the aim of defining a robust computational protocol, a general time-independent formulation to compute different kinds of vibrationally resolved electronic spectra has been generalized to also support photoelectron spectroscopy. The electronic structure data underlying the simulation are computed using different electron propagator approaches. In addition to the more standard approaches, a new and robust implementation of the second-order self-energy approximation of the electron propagator based on a transition operator reference (TOEP2) is presented. To validate our implementation, a series of molecules has been used as test cases. The result of the simulations shows that, for ultraviolet photoionization spectra, the more accurate nondiagonal approaches are needed to obtain a reliable reproduction of vertical ionization energies but that diagonal approaches are sufficient for energy gradients and pole strengths. For X-ray photoelectron spectroscopy, the TOEP2 approach, besides being more efficient, is also the most accurate in the reproduction of both vertical ionization energies and vibrationally resolved bandshapes.

Optimization of highly excited matrix product states with an application to vibrational spectroscopy

“…We note that this approach is also equivalent to the maximum-overlap (MaxO) methods that are commonly applied for electronic structure problems. [78][79][80] If the optimized MPS deviates only slightly from the guess, the test function (denoted as test MPS | Φ test for the DMRG case) can be constructed by choosing the matrix entries in Eq. ( 1) such that all ONVs but one vanish.…”

Astrochemical relevance of VUV ionization of large PAH cations

“…The theoretical method used here to compute the absolute, total cross section does not distinguish among different categories of electronic excitation; they are in principle all included together (except vibronic coupling) and cannot be distinguished in these calculations. More complex techniques involving many-body theory can compute the structure of the ionization cross sections with considerable accuracy (see e.g., Baiardi et al 2017, for a recent review). However, their computational cost would be extremely high for the species considered here, and they are out of the scope of the present work, in which such detailed structure is not resolved in any case.…”

Interplay of Stereoelectronic and Vibrational Modulation Effects in Tuning the UPS Spectra of Unsaturated Hydrocarbon Cage Compounds

“…In this work, the UPS spectra of six semirigid molecules were simulated with the inclusion of vibrational modulation effects by means of a computational strategy recently proposed by some of the present authors . Concerning the electronic computations, good agreement between computational and experimental ionization energies is obtained by both diagonal and nondiagonal approaches.…”

“…Therefore, the main purposes of the present study are (i) the validation of a computational protocol combining methods based on one-electron Green’s functions for the calculation of ionization potentials (IPs) with a characterization of ground electronic states obtained with DFT-based methods and (ii) the integration of the computational results already available in the literature, especially concerning the characterization of the vibrational progressions observed in the experimental spectra. 23 …”