Publications (7)0 Total impact
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ABSTRACT: Density functional approximations fail to provide an accurate treatment of weak interactions. More recent, but not readily available functionals can lead to significant improvements. A simple alternative to correct for the missing weak interactions is to add, a posteriori, an atom pair-wise dispersion correction. We here present a density dependent dispersion correction, dDXDM, which dramatically improves the performance of popular functionals (e.g., PBE-dDXDM or B3LYP-dDXDM) for a set of 145 systems featuring both inter- and intramolecular interactions. Whereas the highly parameterized M06-2X functional, the long-range corrected LC-BLYP and the fully non-local van der Waals density functional rPW86-VV09 also lead to improved results as compared to standard DFT methods, the enhanced performance of dDXDM remains the most impressive.
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ABSTRACT: Bond separation reactions of highly branched alkanes are used to assess (de)stabilizing interactions associated with various 1,3-nonbonded substituent patterns. While n- and singly methylated alkanes show positive bond separation energies (BSEs), which increase systematically along the series, permethylated alkanes are characterized by decreasing BSEs. Analysis shows that singly methylated alkanes are more stabilized than linear alkane chains and that the unique destabilizing feature of permethylated alkanes arises from the close proximity of bulky methyl groups causing highly distorted geometries along the carbon backbone.
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ABSTRACT: The ring strain energies of carbomeric-cycloalkanes (molecules with one or more acetylene spacer units placed into carbon single bonds) are assessed using a series of isodesmic, homodesmotic, and hyperhomodesmotic chemical equations. Isodesmic bond separation reactions and other equations derived from the explicitly defined hierarchy of homodesmotic equations are insufficient for accurately determining these values, since not all perturbing effects (i.e., conjugation and hyperconjugation) are fully balanced. A set of homodesmotic reactions is proposed, which succeeds in balancing all stereoelectronic effects present within the carbomeric rings, allowing for a direct assessment of the strain energies. Values calculated from chemical equations are validated using an increment/additivity approach. The ring strain energy decreases as acetylene units are added, manifesting from the net stabilization gained by opening the C−CH2−C angle around the methylene groups and the destabilization arising from bending the C−C≡C angles of the spacer groups. This destabilization vanishes with increasing parent ring size (i.e., the angle distortion is less in the carbomeric-cyclobutanes than in the carbomeric-cyclopropanes), leading to strain energies near zero for carbon-cyclopentanes and carbon-cyclohexanes.
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ABSTRACT: Density functional approximations fail to provide a consistent description of weak molecular interactions arising from small electron density overlaps. A simple remedy to correct for the missing interactions is to add a posteriori an attractive energy term summed over all atom pairs in the system. The density-dependent energy correction, presented herein, is applicable to all elements of the periodic table and is easily combined with any electronic structure method, which lacks the accurate treatment of weak interactions. Dispersion coefficients are computed according to Becke and Johnson’s exchange-hole dipole moment (XDM) formalism, thereby depending on the chemical environment of an atom (density, oxidation state). The long- range ∼R-6 potential is supplemented with higher-order correction terms (∼R-8 and ∼R-10) through the universal damping function of Tang and Toennies. A genuine damping factor depending on (iterative) Hirshfeld (overlap) populations, atomic ionization energies, and two adjustable parameters specifically fitted to a given DFT functional is also introduced. The proposed correction, dDXDM, dramatically improves the performance of popular density functionals. The analysis of 30 (dispersion corrected) density functionals on 145 systems reveals that dDXDM largely reduces the errors of the parent functionals for both inter- and intramolecular interactions. With mean absolute deviations (MADs) of 0.74-0.84 kcal mol-1, PBE-dDXDM, PBE0-dDXDM, and B3LYP-dDXDM outperform the computationally more demanding and most recent functionals such as M06-2X and B2PLYP-D (MAD of 1.93 and 1.06 kcal mol-1, respectively).
