Automatic Determination of the Non-Covalent Stable Conformations of the NO 2 -Pyrene Cluster in Full Dimensionality (81D) Using the vdW-TSSCDS Approach.

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 NO 2 radical and the pyrene (C 16 H 10 ) 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 NO 2 -pyrene, this network exhibits four major basins which can be characterized by their point-group symmetry. A central one, of global C 2 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 C 1 symmetry. The latter can be distinguished by the projection of the position of the NO 2 nitrogen atom on the pyrene plane in combination with the relative orientation of the oxygen pair pointing either inwards, outwards, upwards or downwards.