© 1998 Rainer Glaser. All rights reserved.
The University of Missouri at Columbia
Chemistry 433 - Computational Chemistry - Winter Semester 1998
Instructions Organizing and Topic Assignments
of the Specific Exercises Related to
Theoretical Level Dependency
"Theoretiocal Level Dependency" always is a central issue in any
theoretical discussion. One needs to know how well a given
model performs. The theoretical level dependency of different properties
can be quite different. Many methods give good geometries, for example,
while there are fewer methods that provide accurate dipole moments.
So, we will take a look at "Theoretical Level Dependency" for a
few cases that exemplify selected issues well. The topics are listed
below. There is one topic per group. If you have a preference for a
certain topic, do let me know as soon as possible.
Here is what you should do. Starting with the semiempirial methods
CNDO, MNDO, AM1 and PM3, carry out the computations using Gaussian94 on
Shiva. Tabulate your results so that the parameters show up on the
horizontal and the theoretical levels on the vertical. We will be adding
data obtained at many theoretical levels as we go along in the course.
Eventually, we will
produce pdf files of your tables and post them on the web. You can
include as many parameters in the tables as you like. Start with the key
parameter I am asking for (e.g. the dipole moment of CO, the rotational
barrier in formamide, ...) and then add parameters that you think are of
interest as well. Feel free to talk to me if you need some feedback. Of
course, it would not hurt to run a CAS search on your topic and learn what
others have thought about the issue before.
I will update this file as we go along specifying additional levels at
which you should examine the problem. Of course, do not feel limited by
my requests! You can do as many theoretical levels as you like (just do
not crash the system).
Update 4/3/98: Carry out calculations at the levels RHF/STO-3G,
RHF/3-21G, RHF/6-31G, RHF/6-31G* and RHF/6-31G**. In all cases, optimize
the structures using the keyword opt=z-matrix. In addition, you should use
the keyword GFP (stands for Gaussian Function
Printout) in these calculations so that you get some idea about the
magnitudes of the exponents. From now on, also please keep track of the
time used for the calculation. Record the computation time given at the
bottom of the output file.
Update 4/13/98: Carry out optimizations at MP2(fc)/6-31G*. Then
also carry out single point calculations at the levels
MP2(fc)/6-31G**//MP2(fc)/6-31G* and MP2(fc)/6-311G**//MP2(fc)/6-31G*.
Make a note of CPU time requirements.
Carry out single point energy calculations at the levels CIS and CISD
using the basis sets 6-31G* and 6-311G**. Use the HF/6-31G* structures.
CI calculations are not "size consistent" and this deficiency is corrected
for in part by an empirical "size consistency correction". List both
the CI energy and the SSC CI energy.
Topics and Group Assignments
Acetonitrile and its isomer, MeCN and MeNC.
Assigned to Group #5 (O-Methylation):
Hongbin & Emma.
These highly polar molecules are very different bonding situations. It
will be of interest to see how well theory can reproduce the isomer
preference energy. Keep an eye also on the lengths of the multiple bond
and the dipole moments.
The dipole moments of CO.
Selected by Group #1 (The Fock-ing Computational Chemists):
Mike Lewis & Graeme Day.
Computations of the dipole moment of CO have a long history. Some
computations do not even get the direction right! Keep an eye also on the
variations of the bond length with changes in the theoretical model.
The rotational barrier in formamide.
Assigned to Group #4 (The Hamiltonians):
Leonid & Lixin.
A classical case. The barrier that governs the conformations of peptides.
You will need to compute the equilibrium structure (which may not be
planar at the amino-group) and two transition
state structures (the ones with a pyramidal NH2 group that have
the N-lone pair either syn or anti with the C=O bond)
at each level.
Comparison of 1,3-pentadiene and 1,4-pentadiene.
Assigned to Group #2 (Nitrosamine):
Wenge & Jianzheng.
One is conjugated and one is not. Does the theoretical model account for
this difference well?
Comparison between propene and cyclopropane.
Selected by Group #3 (The Hueckelberries):
Bruce Flint & Sang Lee.
This is about ring strain, of course.