title: Quantum-classical calculations of X-ray photoelectron spectra of polymers-Polymethyl methacrylate revisited.
abstract: In this work, we apply quantum mechanics/molecular mechanics (QM/MM) approach to predict core-electron binding energies and chemical shifts of polymers, obtainable via X-ray photoelectron spectroscopy(XPS), using polymethyl methacrylate as a demonstration example. The results indicate that standard parametrizations of the quantum part (basis sets, level of correlation) and the molecular mechanics parts (decomposed charges, polarizabilities, and capping technique) are sufficient for the QM/MM model to be predictive for XPS of polymers. It is found that the polymer environment produces contributions to the XPS binding energies that are close to monotonous with the number of monomer units, totally amounting to approximately an eV decrease in binding energies. In most of the cases, the order of the shifts is maintained, and even the relative size of the differential shifts is largely preserved. The coupling of the internal core-hole relaxation to the polymer environment is found to be weak in each case, amounting only to one or two tenths of an eV. The main polymeric effect is actually well estimated already at the frozen orbital level of theory, which in turn implies a substantial computational simplification. These conclusions are best represented by the cases where the ionized monomer and its immediate surrounding are treated quantum mechanically. If the QM region includes only a single monomer, a couple of anomalies are spotted, which are referred to the QM/MM interface itself and to the neglect of a possible charge transfer.
authors.affiliation: Nano and Molecular Systems Research Unit, University of Oulu, P.O. Box 3000, FIN-90014 Oulu, Finland.
authors.forename: T
authors.identifier: 0000000316101431
authors.initials: T
authors.lastname: Löytynoja
authors.affiliation: Division of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, SE-106 91 Stockholm, Sweden.
authors.forename: I
authors.initials: I
authors.lastname: Harczuk
authors.affiliation: Nano and Molecular Systems Research Unit, University of Oulu, P.O. Box 3000, FIN-90014 Oulu, FinlandDivision of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, SE-106 91 Stockholm, Sweden Laboratory for Nonlinear Optics and Spectroscopy, Siberian Federal University, 660041 Krasnoyarsk, Russia.
authors.forename: K
authors.initials: K
authors.lastname: Jänkälä
authors.affiliation: Division of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, SE-106 91 Stockholm, Sweden.
authors.forename: O
authors.identifier: 0000000291238174
authors.initials: O
authors.lastname: Vahtras
authors.affiliation: Division of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, SE-106 91 Stockholm, Sweden.
authors.forename: H
authors.initials: H
authors.lastname: Ågren