A computational analysis of interaction energies in methane and neopentane dimer systems

The gas-phase interaction energies of methane and neopentane dimers are calculated at various intermolecular distances and geometries using several molecular mechanics and semiempirical parameter sets. For comparisons, a set of reference calculations are also performed using the 6-311G (2d, 2p) basis set with the inclusion of second-order Möller-Plesset energies (MP2) and basis set superposition corrections. These calculations are further used to examine the mechanism by which the AMI and PM3 methods account for dispersion interactions in molecular systems. While no specific parameter(s) are included in semiempirical energy functions to capture such effects, the results indicate that both methods produce favorable interaction energies at near contact distances for the dimer systems. AMI energies, however, show much closer agreement with the reference calculations, indicating potential deficiencies in the PM3 parameter set. Although the source of the dispersion energy could be traced to the attractive Gaussians of the core repulsion function in the AMI Hamiltonian, a similar link could not be established for PM3. In contrast, PM3 dispersion energies apparently stem from a collection of contributions implicitly included during parameter optimization, providing no clear mechanism for correction or adjustment. Based on the analysis presented, an approach is also suggested for improving the AMI parameter set.