  We introduce a method for classical trajectory calculations to simulate
collisions between atoms and large rigid asymmetric-top molecules. Using this
method, we investigate the formation of molecule-helium complexes in buffer-gas
cooling experiments at a temperature of 6.5 K for molecules as large as
naphthalene. Our calculations show that the mean lifetime of the
naphthalene-helium quasi-bound collision complex is not long enough for the
formation of stable clusters under the experimental conditions. Our results
suggest that it may be possible to improve the efficiency of the production of
cold molecules in buffer-gas cooling experiments by increasing the density of
helium. In addition, we find that the shape of molecules is important for the
collision dynamics when the vibrational motion of molecules is frozen. For some
molecules, it is even more crucial than the number of accessible degrees of
freedom. This indicates that by selecting molecules with suitable shape for
buffer-gas cooling, it may be possible to cool molecules with a very large
number of degrees of freedom.
