  Accurate and efficient simulation of excited state properties is an important
and much aspired cornerstone in the study of adsorbate dynamics on metal
surfaces. To this end, the recently proposed linear expansion \Delta
Self-Consistent Field (le\Delta SCF) method by Gavnholt et al. [Phys. Rev. B
78, 075441 (2008)] presents an efficient alternative to time consuming
quasi-particle calculations. In this method the standard Kohn-Sham equations of
Density-Functional Theory are solved with the constraint of a non-equilibrium
occupation in a region of Hilbert-space resembling gas-phase orbitals of the
adsorbate. In this work we discuss the applicability of this method for the
excited-state dynamics of metal-surface mounted organic adsorbates,
specifically in the context of molecular switching. We present necessary
advancements to allow for a consistent quality description of excited-state
potential-energy surfaces (PESs), and illustrate the concept with the
application to Azobenzene adsorbed on Ag(111) and Au(111) surfaces. We find
that the explicit inclusion of substrate electronic states modifies the
topologies of intra-molecular excited-state PESs of the molecule due to image
charge and hybridization effects. While the molecule in gas phase shows a clear
energetic separation of resonances that induce isomerization and backreaction,
the surface-adsorbed molecule does not. The concomitant possibly simultaneous
induction of both processes would lead to a significantly reduced switching
efficiency of such a mechanism.
