Accurate Properties for Small Molecules
Methodology
Ab Initio MO Theory
Post-Hartree Fock Methods
Techniques Used
Building, Optimization, Analysis of
Bonding Parameters, Output Analysis
Abstract. You will obtain the dipole moment and
charge distribution for carbon monoxide (CO) at three levels of theory.
You will compare the dipole moment at each level of theory to experiment,
to evaluate the effects of increasing the level of ab initio theory upon
accuracy in predicting this dipole moment.
Procedure. Build the CO molecule with a fixed bond
length of 1.128 Å, the experimental bond length. Carry out a fixed
geometry STO-3G computation, a 6-31G* computation, and an MP2/6-31G* calculation.
Repeat these calculations, but allow the bond length to be optimized in
each calculation. In your Results table, record the computed bond lengths
(where appropriate), charge distribution, and dipole moment for each calculation,
and compare to the experimental numbers listed below. How sensitive is the
dipole moment for this simple molecule to changes in the bond length and
in computational method?
Benchmarks . Experimentally, r(CO) = 1.128 Å
, dipole moment = 0.11 Debye (positive end is on oxygen ). By definition,
the dipole moment m = er, where e = the charge separation, and r is the
distance in angstroms between the centers of the dipole. 1 Debye = 10-18
esu-cm
Results.
| METHOD |
Bond Length |
Carbon Charge |
Oxygen Charge |
Dipole Moment |
| Experimental |
1.128 Å |
-0.0975 |
+0.0975 |
-0.11 Debye |
| STO-3G/RHF |
|
|
|
|
| 6-31G*/RHF |
|
|
|
|
| STO-3G/MP2 |
|
|
|
|
| 6-31G*/MP2 |
|
|
|
|
Copyright by Paul M. Lahti 1995