The theoretical calculation of gasphase reaction rates

The evaluation of kinetic constants for elementary reactions
is crucial in modeling and simulation of chemical processes.
Complex phenomena such as combustion or atmospheric chemistry are
the result of hundreds of elementary interdependent chemical reactions
that occur in the gas phase. Therefore, in order to use models able
to describe and foresee the evolution of these processes, it is
necessary to know the rate of such reactions, their dependence on
temperature and to understand their mechanism.

The theoretical approach to this goal is made of several
steps:


a) determination of the Potential Energy Surface;

b) express the PES by means of a suitable analytic form;

c) evaluation of the kinetic constant;


Solving the Schroedinger equation for a sufficient number
of geometry configurations we obtain a grid of energy values
against the system coordinates, this is the PES.
In order to calculate the kinetic constants we need to
express the PES in analytic form. Many are the functionals
and the models used to fit the energy values.
Finally, the calculation of the constant is made by means of the collision
trajectories integration (within a classic, semiclassic or quantic scheme)
or with the Variational Transition State Theory.