The second virial coefficient for hard models of alkanes and other flexible molecules is determined numerically using a new algorithm which increases the speed of the calculations by a few orders of magnitude. For alkanes with few carbon atoms; linear and branched chains were considered and the effect of branching was analyzed. For linear hard alkanes, the second virial coefficient was computed for chains with up to 600 carbon atoms and the scaling behavior was studied. The effect of changes in the chemical structure of a flexible molecule (i.e., bond length, bond angle) was also studied. A fast and efficient empirical methodology to estimate the second virial coefficient of hard chains is given. This methodology uses ideas of convex belly geometry. It is shown that the proposed methodology yields very good estimates of the second virial coefficient for chains with up to 100 monomer units, but it predicts incorrectly the scaling law. The virial coefficients provided in this work can be useful in the search of an equation of state for hard alkanes, since it is likely that a good equation of state should provide good estimates of the second virial coefficient.