We report a collection of lowest-energy structures of hydrocarbon molecules C n H m (n = 6-18, m = 0 - 2n + 2) within the wide hydrogen chemical potential range. The genetic algorithm combined with Brenner's empirical potential is applied for the search. The resultant low-energy structures are further examined by ab initio quantum chemical calculations. The lowest-energy molecules with several additional low-energy structures are classified to four groups according to their structural motifs and the phase diagram with respect to carbon atom number and hydrogen chemical potential is presented. The results provide useful information for identifying the hydrocarbon molecules in the interstellar medium as well as addressing the hydrocarbon-related nanofragment growth in experiments.