The hexagonal close packed single crystal is investigated through dislocation dynamics simulations for the purpose to produce strain hardening simulations of different initial dislocation densities. Two aspects are discussed: the treatment of the hcp lattice in Cartesian coordinates with the use of periodic boundary conditions and the impact of initial dislocation configuration on the strain hardening curve. The mobilities of dislocations in hcp crystals are not only different for screw and edge dislocations, glide planes are of various natures resulting in different edge dislocation mobility for different planes. At different initial dislocation densities, different magnitudes and width of the yield point have been observed. For small dislocation densities the yield point is extremely large. As the initial dislocation density is increased the point becomes less and less noticeable. This indicates that a simple mechanical tensile test can be used to approximately determine the initial dislocation density in a single crystal.