  The nucleation of particles from trace gases in the atmosphere is an
important source of cloud condensation nuclei (CCN), and these are vital for
the formation of clouds in view of the high supersaturations required for
homogeneous water droplet nucleation. The methods of quantum chemistry have
increasingly been employed to model nucleation due to their high accuracy and
efficiency in calculating configurational energies; and nucleation rates can be
obtained from the associated free energies of particle formation. However, even
in such advanced approaches, it is typically assumed that the nuclei have a
classical nature, which is questionable for some systems. The importance of
zero-point motion (also known as quantum nuclear dynamics) in modelling small
clusters of sulphuric acid and water is tested here using the path integral
molecular dynamics (PIMD) method at the density functional theory (DFT) level
of theory. The general effect of zero-point motion is to distort the mean
structure slightly, and to promote the extent of proton transfer with respect
to classical behaviour. In a particular configuration of one sulphuric acid
molecule with three waters, the range of positions explored by a proton between
a sulphuric acid and a water molecule at 300 K (a broad range in contrast to
the confinement suggested by geometry optimisation at 0 K) is clearly affected
by the inclusion of zero point motion, and similar effects are observed for
other configurations.
