  A hierarchical parallelization has been implemented in a new unified code
PIMD-SMASH for ab initio simulation where the replicas and the Born-Oppenheimer
forces are parallelized. It is demonstrated that ab initio path integral
molecular dynamics simulations can be carried out very efficiently for systems
up to a few tens of water molecules. The code was then used to study a
Diels-Alder reaction of cyclopentadiene and butenone by ab initio string
method. A reduction in the reaction energy barrier is found in the presence of
hydrogen-bonded water, in accordance with experiment.

Chemical Physics;Computational Physics