Open AccessEnergy Ordering of Molecular Orbitals
Author(s) -
Peter Puschnig,
A. Daniel Boese,
Martin Willenbockel,
Matthias Meyer,
Diana Lüftner,
Eva Maria Reinisch,
Thomas Ules,
Georg Koller,
Serguei Soubatch,
Michael G. Ramsey,
F. Stefan Tautz
Publication year - 2016
Publication title -
the journal of physical chemistry letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.563
H-Index - 203
ISSN - 1948-7185
DOI - 10.1021/acs.jpclett.6b02517
Subject(s) - molecular orbital , atomic orbital , molecular orbital theory , complete active space , density functional theory , slater type orbital , molecular physics , valence bond theory , orbital overlap , molecular orbital diagram , physics , orbital hybridisation , chemical physics , chemistry , atomic physics , molecule , computational chemistry , electron , quantum mechanics
Orbitals are invaluable in providing a model of bonding in molecules or between molecules and surfaces. Most present-day methods in computational chemistry begin by calculating the molecular orbitals of the system. To what extent have these mathematical objects analogues in the real world? To shed light on this intriguing question, we employ a photoemission tomography study on monolayers of 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) grown on three Ag surfaces. The characteristic photoelectron angular distribution enables us to assign individual molecular orbitals to the emission features. When comparing the resulting energy positions to density functional calculations, we observe deviations in the energy ordering. By performing complete active space calculations (CASSCF), we can explain the experimentally observed orbital ordering, suggesting the importance of static electron correlation beyond a (semi)local approximation. On the other hand, our results also show reality and robustness of the orbital concept, thereby making molecular orbitals accessible to experimental observations.