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The new mechanism of GMC formation triggered by gravitationally induced phase transition from warm to cold gas is proposed. It is shown that CMSs can form only in the most massive interstellar clouds with masses exceeded about one million solar mass. The values of critical volume density at which the phase transition takes place, critical column density above which molecules can form, and external pressure determine the properties of forming GMCs. Hydrodynamical consideration make possible to study the process of GMC formation in development. It was found that by the proposed mechanism of GMC formation structure and time evolution of GMC complexes, existence of GMCs in central parts of superclouds, and hierarchy of young stellar groups can be explained.
