  The yardstick of new first-principles approaches to key points on reaction
paths at metal surfaces is chemical accuracy compared to reliable experiment.
By this we mean that such values as the activation barrier are required to
within 1 kcal/mol. Quantum Monte Carlo (QMC) is a promising (albeit lengthy)
first-principles method for this and we are now beyond the dawn of QMC
benchmarks for these systems, since hydrogen dissociation on Cu(111) has been
studied with quite adequate accuracy in two improving QMC studies \cite{hog0,
kdd2}. Pt and Cu require the use of pseudo-potentials in these calculations and
we show that those of Pt are less problematic than those for Cu, particularly
for QMC work. This work determines physisorption energies for hydrogen atoms
and molecules on Pt(111). These systems are used as asymptotes to determine
reaction barriers. As such, they must be referred to clean Pt(111) surfaces and
the isolated hydrogen atom or molecule. Previous work gave the activation
barrier to hydrogen dissociation on Pt(111) using the bridging geometry. The
method used is state-of-the-art ({\it ab initio}) QMC. This barrier agrees to
better than chemical accuracy with a recent Specific Reaction Parameter
(SRP-DFT) calculation, fitted to measurement \cite{iran}. The results give the
dissociation barrier for hydrogen on Pt(111) as 5.4 kcal/mol (QMC and 6.2 by
SRP-DFT) with a QMC standard error below 0.99 kcal/mol. This is encouraging for
establishing less well-known benchmark values of industrial reaction barriers
on Pt(111).
