  A recent study of temperature programmed desorption (TPD) measurements of
small n-alkanes (CNH2N+2) from C(0001) deposited on Pt(111) shows a linear
relationship of the desorption energy with increasing n-alkane chain length. We
here present a van der Waals density functional study of the desorption barrier
energy of the ten smallest n-alkanes (N = 1 to 10) from graphene. We find
linear scaling with N, including a nonzero intercept with the energy axis,
i.e., an offset at the extrapolation to N = 0. This calculated offset is
quantitatively similar to the results of the TPD measurements. From further
calculations of the polyethylene polymer we offer a suggestion for the origin
of the offset.
