Functional group corrections to the GFN2-xTB and PM6 semiempirical methods for noncovalent interactions in alkanes and alkenes.

Analytical corrections were developed to improve the accuracy of the PM6 and GFN2-xTB semiempirical quantum mechanical methods for the evaluation of noncovalent interaction energies in alkanes and alkenes. We followed the approach of functional group corrections, wherein the atom-atom pair corrections depend on the nature of the interacting functional groups. The training set includes 21 alkane and 13 alkene complexes taken from the Donchev et al.

Electrostatic penetration effects stand at the heart of aromatic π interactions.

The nature of the interaction in benzene-containing dimers has been analysed by means of Symmetry Adapted Perturbation Theory (SAPT). The total interaction energy and the preference for the dimers to adopt slipped structures are, apparently, consequence of the balance between repulsion and dispersion. However, our results indicate that this only holds when trends are analysed using fixed intermolecular distances.

The PM6-FGC Method: Improved Corrections for Amines and Amides.

Recently, we reported a new approach to develop pairwise analytical corrections to improve the description of noncovalent interactions, by approximate methods of electronic structures, such as semiempirical quantum mechanical (SQM) methods. In particular, and as a proof of concept, we used the PM6 Hamiltonian and we named the method PM6-FGC, where the FGC acronym, corresponding to Functional Group Corrections, emphasizes the idea that the corrections work for specific functional groups rather than for individual atom pairs. The analytical corrections were derived from fits to B3LYP-D3/def2-TZVP (reference).

New Approach for Correcting Noncovalent Interactions in Semiempirical Quantum Mechanical Methods: The Importance of Multiple-Orientation Sampling.

A new approach is presented to improve the performance of semiempirical quantum mechanical (SQM) methods in the description of noncovalent interactions. To show the strategy, the PM6 Hamiltonian was selected, although, in general, the procedure can be applied to other semiempirical Hamiltonians and to different methodologies. A set of small molecules were selected as representative of various functional groups, and intermolecular potential energy curves (IPECs) were evaluated for the most relevant orientations of interacting molecular pairs.

Curvature and size effects hinder halogen bonds with extended π systems.

Curvature and size effects in halogen interactions with extended aromatic species have been evaluated, employing computational methods, in dimers formed by dihalogens Cl2, Br2 and I2 with both planar (coronene and circumcoronene) and curved (corannulene, sumanene and C60) aromatic systems. The main controlling factor in these interactions is dispersion, so they become stronger as the size of the halogen grows. The nature of the interaction with the halogen changes depending on the curvature and the extension of the aromatic system.