Assessing computationally efficient isomerization dynamics: ΔSCF density-functional theory study of azobenzene molecular switching

We present a detailed comparison of the S0, S1 (n -> pi*) and S2 (pi -> pi*) potential energy surfaces (PESs) of the prototypical molecular switch azobenzene as obtained by Delta-self-consistent-field (Delta SCF) density-functional theory (DFT), time-dependent DFT (TD-DFT) and approximate coupled cluster singles and doubles (RI-CC2). All three methods unanimously agree in terms of the PES topologies, which are furthermore fully consistent with existing experimental data concerning the photo-isomerization mechanism. In particular, sum-method corrected Delta SCF and TD-DFT yield very similar results for S1 and S2, when based on the same ground-state exchange-correlation (xc) functional. While these techniques yield the correct PES topology already on the level of semi-local xc functionals, reliable absolute excitation energies as compared to RI-CC2 or experiment require an xc treatment on the level of long-range corrected hybrids. Nevertheless, particularly the robustness of Delta SCF with respect to state crossings as well as its numerical efficiency suggest this approach as a promising route to dynamical studies of larger azobenzene-containing systems. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3664305]