Ab initio basis set

Many conventional and commonly-used ab initio basis sets are supported in HyperChem. These basis sets include ... 

Table 7.10 Calculated (ab initio, semiempirical, DFT) and experimental [111] UV spectra of methylenecyclopropene, wavelength, nm (relative intensity). The recommended ab initio basis set [111] and DFT functional and basis set [110] are used. The ab initio results are from Table 5.16, and the semiempirical results are from Table 6.5...

Tel. 203-432-6288, fax 203-432-6144, e-mail dan rani.chem.yale.edu Plots of wavefunctions in three dimensions from semiempirical and popular ab initio basis sets. Silicon Graphics, Sun, VAX, Cray, and others. 

Fig. 1.3. The dependences on the dihedral angles (0, i/(), of the chemical shielding constants for the L-alanine residue C and Cp carbons in peptides. Chemical shielding calculations were carried out using the GIAO-CHF method with 4-31G ab initio basis set. The 4-31G optimized geometries for the model molecules, A-acetyl-A -methyl-L-alanineamide, were employed (a) the isotropic (b) cru, (c) 0-22 (d) cr, for the C/3 carbon (unit in ppm) (e) the isotropic (f) ctu (g) 0-22 and (h) o-j, for the C carbon (unit in ppm).

Comparison of the various ab initio basis sets has been made to resolve the question of the best basis set for a particular application. It was concluded that of the lower-level basis sets (STO-3G, 4-3IG, STO-3G ), STO-3G provides the best electronic representation of hypervalent phosphorus and sulphur species (Collins et al 1976). Of the higher-level basis sets (3-2IG, 3-21G( ), 6-31G ), it was concluded that the 6-31G and 3-21G( ) sets give good representation of such species (Pietro et al., 1982). A major conclusion of both these studies is that d-orbital functions are essential for calculations on species containing hypervalent second-row elements and bonds involving two heavy atoms. 

The Schrbdinger equation can only be solved approximately, even for the individual building blocks of a DNA or RNA molecule -- the bases, sugars and phosphates. Early efforts necessarily employed semi-empirical methods or minimal ab initio basis sets due to computational limitations. Pioneering work on the application of quantum mechanics for elucidating the chemical and physical properties of nucleic acids was carried out by the Pullmans (73). These early studies laid out most of the basic issues which still concern researchers, including 1) to quantitatively... 

Based on PES calculations on the isolated complexes, the existence of further cyclic ion pairs has been suggested, among them for ammonium salts of zeolite-clusters or ammonium tetrafluoroborate (Ref.46, 3-21G and 6-3IG ab initio basis sets). In these cases, an asymmetric hydrogen bonded form did not occur as a second minimum on the PES. Surprisingly, the proton transfer in the eight-membered formamidine dimer is supposed to pass a transition structure of lower symmetry to form an ion-pair like intermediate... 

From the Raman optical activity of trans-2,3-dimethylthiirane the absolute configuration has been determined. Its experimental spectrum is very similar to that calculated ab initio, basis set 6-3IG ) for the 2R,3R-isomer [5] with best agreement in the skeletal vibration region. 

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