Abstract
<jats:p>Second-order Møller-Plesset theory (MP2) is the simplest wavefunction-based treatment of electron correlation and is a good compromise between accuracy and efficiency for evaluating the strength of many non-covalent intermolecular interactions. It has also been improved in accuracy and robustness by regularized methods, such as the size-consistent second order Brillouin-Wigner perturbation theory (BWs2) and κ-MP2. This work presents a non-technical summary of the recently developed second-generation absolutely localized molecular orbital (ALMO) energy decomposition analysis (EDA) method at the post-SCF level of MP2 and regularized MP2. The so-called frozen term is redefined to ensure that long range correlation contributions to electrostatics are correctly captured. The resulting EDA provides useful basis set limits to the correlation corrections for all physical contributions: donor-acceptor charge transfer, electrical polarization, dispersion, and Pauli repulsions and electrostatic interactions associated with frozen monomer orbitals. EDA calculations using MP2 are reported on several classes of systems: model systems to assess correlation effects on permanent and induced electrostatics, hydrogen bonds between water and model carbohydrate molecules, tetrel bonds ranging from 15 to 90 kJ/mol in strength, two halogen bonds-one of which is known to be solvent resistant, and a pair of “anti-electrostatic” halogen bonds. Several general conclusions emerge. First, to obtain physically reasonable conclusions, it is essential that the correlation contribution is not identified with dispersion alone: large corrections to the frozen term corresponding to changes in electrostatics are observed due to correlation. Second, the fingerprint of intermolecular interactions provided by the EDA reveals a range of interesting trends in the character of intermolecular interactions as a function of their type and strength. For example, the solvent-stable halogen bond is confirmed to be dominated by charge-transfer stabilization. On the other hand, the strongest tetrel bonds studied here are dominated by contributions from polarization, while more conventional hydrogen bonds between water and model carbohydrates represent a synergy between electrostatics, dispersion, and charge transfer. The metastability (or stability) of nominally antielectrostatic interactions depends strongly on the differing distance dependence of attractive and repulsive terms, and we demonstrated tuning based on changes in permanent electrostatics by increasing charge-separation through use of phenyl linkers.</jats:p>