I am a beginner to VB theory, and now need for your kind comments and help.
Questions came when I obtained the dissociation curve of the polar HF molecule, and both the covalent H-F and the major ionic H+F- structure. The VB calculations were perfomed at the L-VBCISD/6-31+G** level, and the wavefunction of the single VB structure was seperately optimized. The minumum of the covalent and ionic curve are -56.2 and -58.8 kcal/mol, respectively. From these results, it seems that the H-F bond has more ionic character, less covalent character.
But, from the hamiltonian matrix and weight of the full VB calculation, the weight of covalent VB structure is larger than 0.5, and I thought that the covalent VB structure should contribute more to the bonding than the ionic VB. But now I was puzzled at the weights of covalent and ionic component, since the above seperate wavefunction optimization suggests a more ionic character.
Any advices and comments are appreciated. A sample of input file with HF at a distance of 0.91 Angstrom is provided.
XMVB Input xmi file
HF molecule,3 structures
frgtyp=sao orbtyp=hao str=full
iscf=2 vbcisd ncor=1 guess=read
1 1 1 1
1 1 1 1 1 1