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Costly Hessian computations were achieved for three purposes :
first to determine the topological nature of geometries on
the Potential Energy Surface, second to take into account the
ZPE, within the adiabatic and harmonic approximations,
and third to compute the vibrational spectra.
We took care of including ZPE contributions in determining the lower
geometry for each species.
However, ZPE contributions did not modify the choice between
stationary points that were under consideration, among possible
candidates for the global minimum geometry.
The lowest electronic energy geometry turned always always to be the
lowest vibronic energy.
No dramatic isotopic substitution effects are therefore envisioned,
at least in the gas phase.
The geometry predictions obtained from
CORINA
are, in general, validated by our MP2 computations.
However, in the two cases of 2-PropylOxonium and 2-ButylOxonium,
do CORINA predictions
appear to be in disagreement with our ab initio computations.
This corresponds to a 75% success rate.
The fact that the IsoButylOxonium cation does not feature any symmetry is going
to unfortunately imply much more costly computations in order to estimate
the electronic absorption spectrum of the corresponding radical.
Vibrational absorption spectra are also reported for each geometry
under consideration and could provide some help to identify molecular
geometries.
Interactive techniques of geometry visualization, such as provided by
the ChemApp Java applet
allow to
understand and investigate geometry structures in a much more convenient
way than 2D static pictures.
One big advantage of a Java applet is that it does not require the reader to have
any helper application or plugin installed in her/his browser.
However, many browsers have the Java compatiblity disabled, and it take times to load the Java interpreter
into memory.
Once the
Java Virtual Machine (JVM) ( see Specs ) is loaded, other visualizations are much quicker to appear.
Another aspect concerning visualization tools
is that a freely available software does not necessarily means that it is
a free software under the GPL
or an open source software.
At the time, in 1999, when our decision was made, we were not conscious enough of the
issues involved in the long term.
There is no guarantee that a software that was freely available
in 1999 will continue to be easily available or updated.
This is a lesson that has been learned.
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