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This new version supersedes all previous versions. Please download it from the MultiWell website:
NEW IN VERSION 2011.1 (March
2011):
1.
MULTIWELL: error found and corrected. It was discovered that we did not implement the
centrifugal corrections correctly. Although we used the approximate centrifugal
correction described by Marcus [1966], the approximate k(E) was not properly
averaged over the thermal rotational distribution. As a result, the following
factor was omitted: C = (I+/I)exp(1-I+/I), where I+ and I are the moments of
inertia of the adiabatic rotors in the transition state and in the reactant
molecule, respectively. For tight transition states (I+ ≈ I), C is nearly equal
to unity, but it can be much smaller than unity for loose transition states.
Thus k(E) was larger than it should have been, which made collisions less
competitive, when all other factors were held constant. This had several
effects: (a) larger values of the energy transfer parameter alpha (average
energy transferred in deactivating collisions) were required in order to fit
experimental data; (b) at low pressures, collisions were less efficient than they
should have been, affecting the rate constant at low pressures; (c) the
relative rates in competitive multi-channel reactions may have been affected to
some extent; (d) the decay of fractional population was faster than it should
have been, leading in some cases to rates greater than k(infinity). The effect
of this error ranges from negligible to significant, depending on the specific
simulation. The last effect was only detectable when k(uni) was large enough
for multiwell to be used effectively for thermal reaction rates (i.e. only for
reactions with very loose transition states at relatively high temperatures).
The error was corrected and new
capability was added: the number of adiabatic rotations can now be selected
(previously it was always assumed that two rotations are adiabatic). For users,
the only change is in the keyword calling for centrifugal corrections. Four
keywords are now recognized: NOCENT (no centrifugal corrections), CENT1
(centrifugal correction assuming one adiabatic rotor), CENT2 (centrifugal correction assuming two
adiabatic rotors; this should be the usual choice), CENTX (legacy centrifugal
correction, which is incorrect and not recommended).
The User Manual will soon be updated to
reflect these changes and to provide a more complete explanation of the
centrifugal corrections. If in the meantime you need more detailed information,
please send a message to [log in to unmask].
2.
THERMO and MOMINERT: enhanced capability: many more elements
and isotopes are now recognized automatically by these programs. The User
Manual will soon be updated to reflect these changes.