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Many questions in physical cosmology regarding the thermal history of theintergalactic medium, chemical enrichment, reionization, etc. are thought to beintimately related to the nature and evolution of pregalactic structure. Inparticular the efficiency of primordial star formation and the primordial IMFare of special interest. We present results from high resolutionthree--dimensional adaptive mesh refinement simulations that follow thecollapse of primordial molecular clouds and their subsequent fragmentationwithin a cosmologically representative volume. Comoving scales from 128 kpcdown to 1 pc are followed accurately. Dark matter dynamics, hydrodynamics andall relevant chemical and radiative processes (cooling) are followedself-consistently for a cluster normalized CDM structure formation model.Primordial molecular clouds with ~10^5 solar masses are assembled by mergers ofmultiple objects that have formed hydrogen molecules in the gas phase with afractional abundance of ~10^-4. As the subclumps merge cooling lowers thetemperature to ~200 Kelvin in a `cold pocket' at the center of the halo. Withinthis cold pocket, a quasi-hydrostatically contracting core with ~200 solar massand number densities > 10^5 cm^-3 is found. We find that less than 1% of theprimordial gas in such small scale structures cools and collapses tosufficiently high densities to be available for primordial star formation.Furthermore, it is worthwhile to note that this study achieved the highestdynamic range covered by structured adaptive mesh techniques in cosmologicalhydrodynamics to date.Comment: 9 pages, 3 figures, ApJ in pres

The Formation and Fragmentation of Primordial Molecular Clouds