A method is developed for simulating undefined multicomponent mixtures such as petroleum fractions, coal liquids, and gas condensates using a preselected set of pure components. The molecularly explicit characterization model (MECM) uses routinely made measurements of a petroleum fraction's bulk properties such as the ASTM D86 distillation, API gravity, Reid vapor pressure (RV-P), and the paraffin, naphthene, and aromatic (PNA) content along with the pure components' properties in an optimization algorithm to define the molecular composition of the petroleum fuel suitable for simulation purposes. The model is tested using a set of 68 pure components to characterize petroleum naphtha and simulate its separation in a distillation column. This model is found to be of better accuracy than the characterization methods currently used. The method has the flexibility to be tailored to use any set of pure components as desired and may further be used to predict the properties of petroleum fractions.