Neopentane and TMS are used as model M(CH3)(4) systems to investigate intramolecular interactions. The nonbonded site-site potential between two proximal hydrogen atoms on different mthyl groups, V-nb(d(HH)), is not Lennard-Jones- or Morse-like but is found to be pseudolinear in hydrogen -hydrogen internuclear separation, dHH, for both neopentane and TMS. The Morse potential is found to be a poor basis in which to expand Vnb(dHH). The nonbonded site-site potential is conformation-dependent and not transferable between molecules. The individual contributions to V-nb(d(HH)) are presented. The local mode parameters for neopentane and TMS are calculated ab initio for a variety of molecular conformations. The ab initio values of the local mode frequency and local mode anharmonicity are increasingly blue-shifted with increasing steric hindrance. Electron correlation is found to be increasingly important with decreasing internuclear separations, dHH.