Automatic Determination of the Non-Covalent Stable Conformations of the NO₂-Pyrene Cluster in Full Dimensionality...

We present the detailed topographical characterisation (stationary points and minimum energy paths connecting them) of the full dimensional (81D) intermolecular potential energy surface associated with the non-covalent interactions between the NO₂ radical and the pyrene (C₁₆H₁₀) molecule. The whole procedure is (quasi) fully automated. We have used our recent algorithm vdW-TSSCDS as implemented on the freely-available AutoMekin software package. To this end, a series of inexpensive classical trajectories using forces from a low-level (semi-empirical) theory are used to sample the configuration space of the system in the search for candidates to first order saddle points. These guess structures are determined by means of a graph-theory based algorithm using the concept of adjacency matrix. Low-level optimizations are followed by re-optimizations at a final high-level of theory (DFT and CCSD(T)-F12 in our case.). The resulting set of stationary points and paths connecting them constitutes the so-called reaction network. In the case of NO₂-pyrene, this network exhibits four major basins which can be characterized by their point-group symmetry. A central one, of global C₂ symmetry, comprises the global minimum (as well as all other permutationally related conformers) together with the corresponding C_2v saddle points connecting them. This central basin is connected to three others of lower C₁ symmetry. The latter can be distinguished by the projection of the position of the NO₂ nitrogen atom on the pyrene plane in combination with the relative orientation of the oxygen pair pointing either inwards, outwards, upwards or downwards.