MU - Glaser - Chem433 - WS98 - Theoretical Level Dependency

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.

Update 4/17/98: 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

Topic #1: 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.

Topic #2: 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.

Topic #3: 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.

Topic #4: 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?

Topic #5: Comparison between propene and cyclopropane.
Selected by Group #3 (The Hueckelberries): Bruce Flint & Sang Lee.

This is about ring strain, of course.