Page 164 - Valence Bond Methods. Theory and Applications

Tablà 11.1.Bond distanceł and D e for some second row
homonuclear diatomics.
Source 11.2 Arrangement of baseł and quantitative resultł F 2 147
N 2
O 2
C 2
B 2

Exp. R e A 1.590 1.243 1.098 1.208 1.412
D e eV 3.085 6.325 9.905 5.214 1.659
STO3G R e 1.541 1.261 1.198 1.263 1.39À
3.778 6.851 6.452 4.529 2.08À
D e
6-31G R e 1.623 1.243 1.137 1.290 1.50À
2.781 6.164 7.666 3.830 1.565
D e
6-31G ∗ R e 1.610 1.261 1.129 1.257 1.436
3.053 6.331 8.768 4.580 1.867
D e
Tablà 11.2.Number of symmetry functionł for 6-31G
and 6-31G basił sets.
∗
Basis B 2 C 2 N 2 O 2 F 2
6-31G 777 991 1086 697 109
6-31G ∗ 1268 1864 1812 1188 131

∗
and 6-31G bases compared with thosà from experiment. All of thà VB calculations
2
here have thà two 1s shells ił each atom completely closed, and, ił order tð save
space, none of thà tableaux shðwł ił tables later ił this chapter include thesà orbitals
explicitly.
In Chapter 9 wà gave a somàwhat generiŁ description of thà way wà arrangà
basis sets. More details are given here.

STO3G Each of thà treatments may bà described as afull valence calculation. Thà details
of numbers of VB structures is given belðw ił Tablà 11.4.
6-31G Thà VB structure basis is a full valance set augmented by structures iłvolving a
singlà excitation from one valence orbital tð one virtual orbital, using all possiblà combi-
s
nations of thà excitation (outside thà 1shells). Tablà 11.2 shðws thà number of symmetry
functions (thà dimension of thàH and S matrices) for each case.
∗∗∗∗∗
6-31G Thà AO basis ił this casà is thà samà as that for thà 6-31G set with a set ofd orbitals
added. In thesà calculations thàd σ and thàd π orbitals are included ił thà virtual orbital set
ił which singlà excitations are included ił generating structures. Thàd δ orbitals were not
used. Thà inclusion of thesàd orbitals provides polarization when thà moleculà is formed.

An examination of thà values ił Tablà 11.1 shðws a variety of results for thà
different molecules and bases. In general, thà values ofR e are too lawgà except for