  First principles calculations based on density functional theory are having
an incerasing impact on our understanding of molecule-surface interactions. For
example, calculations of the multi-dimensional potential energy surface have
provided considerable insight into th edynamics of dissociation processes.
However, these calculations using a plane-wave basis set are very compute
expensive if they are to be fully converged with respect to the plane-wave
energy cutoff, k-point sampling, supercell size, slab thickness, etc. Because
of this, in this study, we have implemented a mixed-basis set approach which
uses pseudo-atomic orbitals and a few low-energy plane waves as the basis set
within a density functional, pseudopotential calculation. We show that the
method offers a computationally cheap but accurate alternative. The energy
barrier for hydrogen dissociation on Cu(111) is calculated as an example.
