The virus/host fusion event of HIV is believed to be predominantly mediated by the viral envelope glycoprotein gp41. The N-terminal fusion peptide of gp41 penetrates the host cell membrane which is required for fusion, however, many details of this event are poorly understood including the secondary structure of the fusion peptide when inserted into the membrane. We are using molecular modeling tools to simulate how fusion peptides interact with lipid membranes and test several proposed models for secondary structure in this region. Specifically, models of gp41 in an explicit POPC lipid bilayer are being constructed and all-atom molecular dynamics simulations are being used to test the stability of various structures including a coiled-coil alpha helix, different β-sheet arrangements, and a β-barrel. Simulations will be used to predict the most likely fusion peptide structure(s) as well as potentially eliminate certain structures as implausible