An efficient workflow for generation of conformational ensembles of density functional theory quality: dimers of polycyclic (hetero-)aromatics

Abstract

The determination of geometries and relative energetics (binding as well as free energies) of ensembles of dimers, as well as for small n-mers, is an important property in physical chemistry, connected to understanding both properties and spectroscopic measurements. In this tutorial review, a workflow for generating conformational ensembles is presented and highlighted for several homodimers. The workflow involves six steps: (i) generate an initial ensemble, using the Conformer-Rotamer Ensemble Sampling Tool (CREST), and its underlying GFN2-xTB method; (ii) reoptimize each member of the ensemble using B97-3c; (iii) discard duplicates; (iv) reoptimize the remaining conformers using ωB97X-D4/def2-SVP; (v) if needed, discard any duplicate conformers; (vi) compute vibrational frequencies using ωB97X-D4/def2-SVP and final single point energies using ωB97X-V/def2-QZVPP. B97-3c was selected for step (ii) due to its performance in a screening of several composite density functional theory (DFT) methods, namely HF-3c, B97-3c, PBEh-3c, r²SCAN-3c, and ωB97X-3c, for the pyrene homodimer. The six-step workflow allows the generation of large DFT-quality ensembles efficiently, as demonstrated on the known pyrene dimer ensemble, and then applied to the homodimers of eight small polycyclic (hetero-)aromatic molecules related to asphaltenes: anthracene, phenanthrene, fluorenone, dibenzofuran, dibenzothiophene, dibenzothiophene oxide, N-methylcarbazole, and benzo[h]quinoline. Methods are suggested for analysis of trends in dimerization structures and energies for these monomers, including an analysis revealing a strong dependence of binding energy on the magnitude of dipole cancellation.