JCP_formic Abstract
Abstract
Double proton transfer in the formic acid dimer has been investigated
with Car-Parrinello ab initio molecular dynamics calculations.
The electronic structure of the dimer has been obtained using
gradient-corrected density functional theory based on the
B-LYP (Becke exchange [Phys. Rev. A 38, 3098 (1988)]
and the Lee-Yang-Parr correlation [Phys. Rev. B 37, 785 (1988)]
functional. The optimized equilibrium and saddle point geometries,
obtained by simulated annealing, are in good agreement with previous
ab initio quantum chemical predictions and experiment.
Thermal and quantum fluctuations of nuclei along the double proton
transfer reaction path have also been investigated at T=300 K.
Thermal fluctuations give a broad distribution of nuclei around the
minimum energy path on the potential energy surface. Quantum fluctuations,
investigated using ab initio path integral molecular dynamics, make the
distribution even broader around the equilibrium structure and cause
the distribution to deviate appreciably from the minimum energy
path on approaching the reaction barrier. In particular, the system
passes through higher energy regions than the geometrical saddle point by tunneling,
an observation which is consistent with the conventional understanding of heavy-light
mass combination reactions. While there is asynchronous movement of the
two protons around the equilibrium structure, synchronous movement becomes
relevant on approaching the reaction barrier.