We are going to compare the effects of the basis set on the energy of a RHF calculation on water.
To speed things up, we are going to use the geometry we found for RHF/3-21G and use that for our calculations.
In Avogadro, open the output file from the RHF/3-21G geometry optimisation of water (remember to rename the file to .gamout), and create several input files for a single-point RHF calculations with increasing basis set size.
The notation for calculations of this type uses the // notation; for example, RHF/6-31G//RHF/3-21G.
When they have all been created, run each of them in turn.
(Note to self: Is there time to run an MP2/3-21G calculation?)
For each calculation, note the final RHF energy and also the wall clock time (found at the end of the output file).