Photoemission Spectroscopy of Organic Molecules Using Plane-Wave/Pseudopoential Density Functional Theory: A Comprehensive Computational Protocol for Isolated Molecules, Molecular Aggregates and Periodic Systems

The possibility to perform low-pressure gas-phase photoemission experiments has allowed the exploration at molecular scale of the complex correlation between electronic and structural properties of the matter. A significant advantage in the removal of the interference of the surrounding at low-pressure is also the easier correlation of the experimental results with theoretical ab-initio simulations. In the context of core ionization, the development of accurate methods for calculating inner-shell binding energies (BEs) offers a dual benefit. Firstly, it aids significantly in interpreting experimental data, helping to assign the contributions of all non-equivalent atoms, even in unresolved features arising from a molecular structure. Secondly, it allows for the anticipation of experimental results by accurately predicting spectral lines. In this context, we have developed and extensively tested a computational protocol based on plane-wave/pseudopotential density functional theory (PW-DFT) to predict XPS BEs in molecules and molecular aggregates. This protocol is based on a ∆SCF approach and has been tested comparing the present theoretical results with experimental XPS spectra collected from several molecular systems in gas phase, ranging from benzene derivates to biomolecules. Our calculations have been performed using semilocal and global/range-separated hybrid density functionals, containing increasing fractions of Hartree-Fock exact exchange (EXX). Specifically, PBE, B3LYP(20 % EXX), HSE(range separated with 25 % EXX at short range ) and BH&HLYP (50 % EXX) have been used for the assessment of the computational protocol. EOM-CCSD has been employed as reference method for comparison. Our PW-DFT approach demonstrated to be generally accurate and robust even using semilocal DFT, and also suitable for application to very large molecular systems and to organic thin films deposited on inorganic surfaces.