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Science Forum Index » Physics - Research Forum » Theoretical Estimation of Chemical Reaction Rate Constants
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| Author |
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| Squark |
 Posted: Wed Jun 28, 2006 5:57 pm |
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Guest
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Hello everyone.
I'm interested in the methods available for estimatating the rate
constants of chemical reactions theoretically. I'm familiar with the
following:
1) In principle the Bohr-Oppenheimer approximation yields the potential
energy of a cluster of atoms as a function of nucleus locations. The
energy can be evaluated in any given point using computational methods
such as Hartree-Fock with configuration interaction, Moller-Plesset
perturbation theory etc. This is not realistic for large molecules,
however.
2) The energy of a molecule in given conformation can be approximated
using standard methods of molecular dynamics, which means a harmonic
oscillator approximation for dependence on bond lengths and angles or
treating some/all of those as constrained to their optimal values, a
similar simplistic approach to dependence of dihedral angles +
electrostatic, hydrogen bond and van der Waals terms. This approach is
not applicable to evaluation the "offshell" energy, i.e. it assumes the
covalent bonds are not formed or broken (with rare exceptions such as
allowing some bonds, e.g. disulfide, to break/form back).
3) Given a potential energy surface, the rate of transitions between
local minima can be estimated using transition state theory, the
reactive flux method, Kramers theory etc.
I'm interested in
A) Computational methods available to attack the problem, including
methods with are viable for comparatively large molecules.
B) Models that are as mathematically simple as possible that still
yield reasonable results.
In principle, 1+3 provides a method but only for small molecules (so it
is not a sufficient answer for A) and is very hard to handle
analytically ("pencil on paper") or to use for building an intuitive
understanding (so it is a poor answer for B).
Thx a lot for any help!
Best regards,
Squark |
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