  We report on a molecular simulation method which captures the self-assembly
of cationic lipid-DNA (CL-DNA) gene delivery complexes. Computational
efficiency required for large length- and time-scale simulations is achieved
through a coarse-grained representation of the intra-molecular details, and via
inter-molecular potentials, which effectively mimic the hydrophobic effect {\em
without} explicit solvent. In addition to showing spontaneous self-assembly of
complexes, the broad utility of the model is illustrated by demonstrating
excellent agreement with X-ray diffraction experimental data for the dependence
of the spacing between DNA chains on the concentration of CLs. At high
concentrations, the large electrostatic pressure induce the formation of pores
in the membranes through which the DNA molecules may escape the complex. We
relate this observation to the origin of recently observed enhanced
transfection efficiency of lamellar CL-DNA complexes at high charge densities.
