counterparts. You are to model silicon clusters, or Si
n
(n = 1 to 10). We will use a trick to
get the geometries from PM3, but then the energies and orbital information from B3LYP
(DFT).
For each n do the following.
1.
Create your Si
n
cluster and optimize using PM3. It helps to start with a geometry as
close as a ‘sphere’ as possible. You may need to change “OPT” to
“OPT(MaxCycles=100)”.
2.
Run a “Molecular Energy” calculation using the geometry from step 1 at the
B3LYP/6-31G(d) level.
3.
Determine the cluster binding energy (Si
n
à
nSi).
4.
Determine the cluster addition energy (Si
n-1
+ Si
à
Si
n
).
5.
Using the geometry from step 2, run a “Molecular Orbitals” calculation and
determine the HOMO-LUMO energy difference. Recall that this is roughly the lowest
energy required to excite an electron. This value is particularly important for
electronic compounds where flow of electrons is common.
Make a plot of cluster binding energy, cluster addition energy, and HOMO-LUMO
difference as a function of n. Do you see any trends? Any comments on your data?

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