Week #5 (Due Sunday, Feb. 15)

 

 

In Spartan complete the following:

a.     Build the water molecule.

b.     Determine the equilibrium geometry of water using the Hartree-Fock method and the STO-3G basis.

c.     Record the # of cycles required to finish (Display à Output)

d.     Record the # of basis functions used.

e.     Record the dipole moment (DisplayàProperties)

f.      Repeat for all the basis sets available to you.

g.     Tabulate the results and compare to experiment.

                                                                       i.     Which basis performed the best?

1.     STO-3G. But it overestimates the dipole moment for other heteroatomic systems such as silyl chloride. This lack of consistency suggests significant basis set errors. The Hartree-Fock methods consistently overestimate the dipole moment even for larger basis sets. This is due to a lack of correlation of the electrons (i.e. the mean field theory).

                                                                        ii.     How many basis functions are used for each atom in STO-3G?

1 for each H 1s electrons and 5 for oxygen (1 for the1s electrons, 1 for the 2s electrons, 3 for four p electrons) for a total of 7

                                                3-21G(*): Basically doubles the basis set size

2 for each H 1s electron and 9 for oxygen (1 for the 1s electrons, 2 for the 2s electrons and 6 for four p electrons

                                                                          iii.     Is the calculation converging as a function of basis set size?

 

As seen in both plots (w and w/o the STO-3G results) the R-squared values suggest no simple relation between basis set size and the calculated dipole moment. The calculation is converged to within +/- 10% for higher basis set sizes. The variation in the dipole as we increase the basis set size indicates the effects of basis set qualities on observables for smaller sized basis sets. For example, note the dipole increases from 6-31G* to 6-311G* and decreases from 6-31G* to 6-31G** although in both cases the basis set increased. In the first case, we are only changing the basis set for the oxygen and in the latter adding polarization to the hydrogen. More appropriate to compare STO-3G to 3-21G to 6-31G* to 6-311+G** and not the intermediates.

 

 

 

 

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