Bastiaan J. Braams
Emory University, Atlanta, GA, USA

Ab initio molecular dynamics on a fitted potential energy surface with
applications to H3O2(-), H5O2(+), and the C3H3+O reaction.



The talk will describe the construction and use of global fits for the
potential energy surface of several molecular systems.  Part of the
novelty of our approach, and key to the success, is to use a
functional form that is invariant under the complete symmetry group of
permutations of like nuclei.  This is technically quite difficult if
one goes beyond the first few orders in an expansion, and we rely on
the mathematical theory of invariants of finite groups and on a
computational algebra system to help generate the codes.  The accuracy
of these global fits is superb.  For example, for H5O2(+) all the way
up to dissociation into H2O and H3O+ (at energy around 120000/cm) we
have an rms error in the fit less than 40/cm.  The fitted potential is
evaluated on a millisecond timescale, so we can do many long MD or QMC
calculations at essentially ab initio accuracy without anywhere near
the cost that is normally associated with ab initio MD, or even with a
Car-Parrinello treatment.  As shown by our treatment of C3H3O
(including six different break-up and reaction channels) the work is
immediately relevant to the evaluation of ab initio cross-sections for
reactions in combustion, as well as for the evaluation of molecular
spectra.