· The calculated frequencies can be compared with experimental results, to give an indication of the accuracy of the calculation.
· In this case,
the calculated frequencies don’t agree well with experiment (n1, Calculated frequency: 2169cm-1 vs.
experimental value 1595 cm-1;
n2 calc. 4141cm-1 vs
expt. 4070 cm-1; n3 calc. 4392 cm-1 vs. expt. 4188cm-1).
· So this
particular method (Hartree-Fock with a STO-3G basis set) doesn’t give great
results for water.
· It also gives
a minimum energy geometry which is quite different from the real
structure.
· However, it
does get the vibrations in the right energy order.
· This is a ‘quick
and dirty’ method - it only took 19 seconds to calculate the frequencies.
· Better
calculations would give better results.
· Good
calculations can be used to help analyse spectroscopic experiments.