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

Theory LevelBond Length (H-Cl) Ň
PM31.267551
AM11.283568
321-G1.293446
631-G1.295263
DZV1.294337


The experimental value for HCl is 1.275 Ň.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 that the HOMO orbital of HCl is non-bonding, and from this model a person can predict how a molecule will react.  
Dipole moments at each level of theory for HCl, experimental 1.109 D2
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 gets 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, which is the bigger basis set, has a lower potential.  Click on graphs to enlarge.  

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.

Methanol                 Aniline

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