Conformer populations, man-square chain dimensions, and end-to-end distance distributions for polymethylene (PM) chains, H(CH2)(n)H with n = 13, 44, and 100, in melts and in various phantom chains have been studied using a parallel computational method of molecular (and Brownian) dynamics simulations with a well-calibrated explicit atom force field. The specific volume-temperature properties of the simulated melt systems are in excellent agreement with experiments. Conformational characteristics of PM chains in melts are identical to those of 1,5 phantom chains, characterized by short-range intramolecular interactions between the atoms separated by up to four skeletal bonds, which correspond to unperturbed chains in Theta solvents according to recent implicit solvent model results of Sariban et al. and Destree et al. Moreover, a Gaussian approximation of end-to-end distance distributions is found to be valid only for long PM chains with n greater than 100. According to our simulation results, the characteristic ratio C-infinity for unperturbed polyethylene chains is found to be ca. 7.9 at 413 K. This value is larger than the generally accepted value? 6.8 (derived from intrinsic viscosity data for relatively polydisperse polyethylene samples), but is corroborated by recent small angle neutron scattering results (7.8 +/- 0.4) for melt chains of nearly monodisperse samples. The fact that; PM chains exhibit identical conformational characteristics in melts, 1,5 phantom chains, and Theta solvents is attributed to the lack of any correlation between local chain conformations and (polymer-polymer or polymer-solvent) intermolecular attractions. Departures from such an ideal behavior seen for other polymers are also discussed.