Abstract
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. Then, analytical corrections to PM6 were derived from fits to B3LYP-D3/def2-TZVP reference–PM6 interaction energy differences. IPECs provided by the B3LYP-D3/def2-TZVP combination of the electronic structure method and basis set were chosen as the reference because they are in excellent agreement with CCSD(T)/aug-cc-pVTZ curves for the studied systems. The resulting method, called PM6-FGC (from functional group corrections), significantly improves the performance of PM6 and shows the importance of including a sufficient number of orientations of the interacting molecules in the reference data set in order to obtain well-balanced descriptions.
Highlights
IntroductionOne of the well-known problems inherent to semiempirical quantum mechanical (SQM) methods is the poor performance in describing noncovalent interactions.[1,2] Over the last years, much effort has been devoted to improve the accuracy of SQM methods for noncovalent interactions, those based on the neglect of diatomic differential overlap (NDDO) approximation.[3,4] The most common strategy used to ameliorate the performance of SQM methods in calculations of intermolecular interactions has been the inclusion of empirical corrections.[5−21]Ř ezać ,̌ Hobza, and their co-workers developed several generations of corrections for dispersion,[6,7,9] hydrogen bond,[7,9] and halogen bond[8] interactions and parameterized them within the PM6 method[22] as well as for other SQM methods
With the corrections obtained from the best fits, we evaluated the intermolecular potential energy curves (IPECs) for the 16 orientations of the formic acid dimer, and they are plotted in Figures S11 and S12, together with the B3LYP-D3 curves and those determined with the parameters of Table 1 (5 atom types)
We have presented a new strategy, that is, the PM6-Functional Group Corrections (PM6-FGC) method, to develop analytical corrections for semiempirical quantum mechanical methods, aimed at improving the description of noncovalent interactions
Summary
One of the well-known problems inherent to semiempirical quantum mechanical (SQM) methods is the poor performance in describing noncovalent interactions.[1,2] Over the last years, much effort has been devoted to improve the accuracy of SQM methods for noncovalent interactions, those based on the neglect of diatomic differential overlap (NDDO) approximation.[3,4] The most common strategy used to ameliorate the performance of SQM methods in calculations of intermolecular interactions has been the inclusion of empirical corrections.[5−21]Ř ezać ,̌ Hobza, and their co-workers developed several generations of corrections for dispersion,[6,7,9] hydrogen bond,[7,9] and halogen bond[8] interactions and parameterized them within the PM6 method[22] as well as for other SQM methods. Contributions to this series of generations were made by Korth[10] and Jensen and co-workers.[11] The final version of this series of corrections is called D3H4X, in reference to the third-generation dispersion correction, fourthgeneration hydrogen-bonding correction, and halogen-bonding correction In this version, the dispersion correction is the D3 proposed by Grimme et al for density functional theory (DFT),[23] but without including the 1/r8 term, which was considered to yield no significant improvement in the case of SQM methods.[9] For these methods, Ř ezaćand Hobza found a specific error in the description of interactions between hydrocarbons, namely, the overestimation of interaction energies and the underestimation of equilibrium distances.[9] To solve this problem, they included a repulsive term for all pairs of hydrogen atoms. The D3H4X correction and other generations of corrections have been implemented in the MOPAC2016 program.[24]
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