Hydrogen Chloride
Models were made using the best ab initio level of theory, which was double zeta valance, DZV.  This is the best level because it is the highest basis set of the molecule.  
 
HCl Bond Length at ab initio level DZV, double zeta valance. 
Bond Length of HCl was found using the Hamiltonians and ab initio levels



The experimental value for HCl is 1.275 Å.1

HCl is a linear molecule.



 
HCl HOMO orbital
The HOMO orbital is the highest energy molecular orbital occupied by electrons.   From the model, it is seen as non-bonding and from this can predict how a molecule can react.  
Dipole moments at each level of theory for HCl2
Theory Level Dipole (D)
PM3 1.379276
AM1 1.383985
321-G 1.856319
631-G 1.875969
DZV 1.897545

The vibrational frequency at the DZV level for HCl was calculated as 2897.419922 cm-1.  The experimental value for HCl from the NIST website was 2990.9 cm-1.3
Potential of bond stretching for the three ab initio levels of theory
The graphs below confirm the DVZ is the best level of theory.  The graph on the left shows all three levels of theory, with increasing theory level the potential energy is lower.  However, from the left graph levels 631-G and DZV look the same.  Looking at a closer view, the graph on the right, it shows that in fact DZV has a lower potential, which is the bigger basis set.
PE surface vs bond lengthPE Surface vs bond length 2
 References:
(1)  http://cccbdb.nist.gov/ .  Geometries. Experimental geometry data for a given species.  HCl.
(2)  Lide, D. R. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, 1992.
(3)  http://cccbdb.nist.gov/. Vibrations. Experimental vibrational data for a given species. HCl. 
Based on template by A. Herráez as modified by J. Gutow
Page skeleton and JavaScript generated by export to web function using Jmol 11.6.6 2008-09-20 22:06 on Mar 18, 2009.