Basis sets results

Fig. 1. Dependence of the vicinal F-F coupling constant in difluoroethyne on the basis set used for the carbon atoms Deviation (in Hz) of the locally dense basis set result from the calculation with the apTJ/apTJ basis set. Only the basis sets on the non-coupled atoms are used as labels on the basis set axis. The apTJ basis set was used for fluorine in all calculations.

In this chapter we will also focus on the dipole moment of molecules. With these, some of the most interesting phenomena are the molecules for which the electric moment is in the wrong direction insofar as the atomic electronegativities are concerned. CO is probably the most famous of these cases, but other molecules have even more striking disagreements. One of the larger is the simple diatomic BF. We will take up the question of the dipole moments of molecules like BF in Chapter 12. In this chapter we will examine in a more general way how various sorts of structures influence electric moments for two simple cases. For some of the discussion in this chapter we restrict ourselves to descriptions of minimal basis set results, since these satisfactorily describe the physics of the effects. In other cases a more extensive treatment is necessary. 

Data are provided in Table 9-4. Hartree-Fock calculations have been limited to STO-3G, 3-2IG and 6-3IG basis sets, and local density, density functional and MP2 calculations have been limited to the 6-3IG basis set. Results from AMI and PM3 semi-empirical calculations have also been provided. MNDO calculations failed to locate reasonable transition states. 

In a comment on this work,39 Close expresses doubt regarding the DFT calculations that indicate a lack of planarity in pyrimidine radical anions, contrary to what is experimentally found in low temperature single crystal studies. He notes that using an extended basis set results in reasonable agreement between the experimental and calculated values of hyperfine coupling constant for the a-proton without the need to move the proton significantly out of the... 

A somewhat more common approach is one that does not try explicitly to extrapolate to the HF limit but uses similar concepts to try to correct for some basis-set incompleteness. The assumption is made that the effects of orthogonal increases in basis set size can be considered to be additive (a substantial amount of work suggests that this assumption is typically not too bad, at least for molecular energies), and thus the individual effects can be summed together to estimate the full-basis-set result. This is best illustrated by example. Consider... 

AX4. BF4 and CF4. The only calculation75 found on BF4 involved a rather small basis set SCF study. Although several papers have dealt with CF4 at a minimal basis set level, the more extended basis set results of Brundle et al.76 dementi et al.,77 and Adams and Clark78 have not been improved upon. 

The reaction of BF3+HF has been looked at from a different viewpoint by Silla et a/.105 SCF calculations using both minimal and 4-31G basis sets were carried out and the molecular geometry optimized. For BF4 the results were similar for the MBS case to those found earlier by Fitzpatrick.108 The geometry of the adduct HBF4- was determined. The 4-31G basis set results gave longer bond lengths. Differences in the... 

Several numerical tests and detailed comparisons of MEDLA electron densities to electron densities computed by traditional ab initio SCF technique using 3-21G and 6-31G basis sets have shown [67,71] that the MEDLA results are invariably of better quality than the standard 3-21G ab initio results, and the MEDLA results are virtually indistinguishable from the standard ab initio 6-31G basis set results obtained with the traditional Hartree-Fock method. 

Equilibrium geometries calculated at the split valence level almost always show shorter bond distances than the minimum-basis-set results and are generally in better agreement with experiment. Split valence wave functions also give considerably better electron distributions, as evidenced by dipole moments and molecule-atom electron-density differences that compare better with experiment (Hehre et al., 1986 Szabo and Ostlund, 1989). 

We calculated the electronic structure of 1-D, 2-D, and 3-D silicon clusters using (DV)-Xa MO method. The calculated energy gap Eg) between HOMO and LUMO decreases with increase of the cluster size and the number of dimensions of the cluster. It is found that including silicon 3d orbitals as basis sets results in lowering the Eg values by about leV. The results also show that the components of silicon 3d orbitals in the unoccupied levels near LUMO axe over 50%. In the case of silicon clusters, the size effect on the Eg value is well described by the quantum-well(QW) model in which the delocalized wavefunc-tions are confined in the potential well. The calculated results predict that Eg... 

---