The evaluation of the ONIOM-EE method for the QM/MM hybrid modeling of HF, CO and CO/HF Clusters

[摘要] ENGLISH ABSTRACT: Quantum mechanics is the method of choice when it comes to the accurate modeling of singlemolecules and clusters. The correlation energy is the single most important aspect whenstudying clusters computationally, and reproducing the correlation energy accurately poses abigger challenge to the computational chemist than in the modeling of single molecules. Veryhigh levels of theory and large basis sets need to be used.Nevertheless, since the calculation of large systems, such as crystals and biologicalsystems, is generally beyond the capacity of quantum mechanics, molecular mechanics isgenerally used for these systems. Unfortunately due to its nature, molecular mechanics cannotmodel important quantum effects, but this problem can be solved by a hybrid system in whichone part of the system is treated by quantum mechanics and the remaining part by molecularmechanics.In order to combine quantum mechanics with molecular mechanics one needs to optimizethe parameters for the molecular mechanics part to allow it to function with the quantummechanics. The research described in this work is based on the ONIOM-EE method, which issuch a hybrid method.In this work we investigate the applicability of the ONIOM-EE method in modelinghydrogen fluoride, carbon monoxide and CO/HF clusters. Most of the clusters' geometries inthis work are not experimentally or computationally known. We therefore perform acomputational analysis of all of the clusters by using various methods including Atoms inMolecules, Natural Bond Orbital analysis, Mulliken population analysis and the analysis ofdelocalized molecular orbitals to obtain information for the development of hybrid systems.During this process we look at different charge derivation schemes and at two differentmethods of optimizing force field parameters for these clusters. We develop a method tomake force field optimization faster and better for specific hybrid systems. This methodshowed that in all cases the optimized parameters were an improvement on those of theUniversal Force Field. We show the importance of an accurate description of the electrostaticinteractions in HF, CO and CO/HF clusters and that this is the Achilles heel when attemptingto optimize van der Waals parameters for force fields. We further show that atomic pointcharges are not a good approximation of a molecules' charge density in hybrid methods. Inaddition, we make suggestions on how the present method for ONIOM-EE can be improvedto make the modeling of van der Waals clusters feasible.

[发布机构] Stellenbosch University