Basis set improvement

Formulas 21.1 through 21.3 are designed for Hartree-Fock wave functions. There are some attempts to define similar indices using wave functions obtained via methods including electron correlation [19]. Similarly, to the situation with respect to basis set improvement, the results based on correlated wave functions do not necessarily make the qualitative picture of bonding easier to understand. An exception is when there is a significant nondynamical correlation in the system,... 

In a previous paper on hydrogen peroxide, the pseudopotential was determined with the A algorithm. The aim of this paper was the evaluation of the basis set effect on the levels. In the present paper, the old calculations have been repeated with the optimal basis set improving the optimization of the internal coordinates. It has to be remarked from the results in Tables, 1 and 2 that the B and C algorithms lead to similar results whereas the A algorithm gives the lowest results for the pseudopotential. V is unappreciated when is determined with the A algorithm. 

As mentioned earlier in this section, the convergence behavior of computed properties generally becomes less exponential as the quantities become less related to energies. By way of illustration, neither the application of the counterpoise procedure nor the addition of diffuse functions to the basis set improves the convergence behavior of the computed anharmonicities co x of the HF molecule (see Figure 10). Even in this case, however, both the uncorrected and corrected curves appear to be converging to the same limits. 

In the section that follows this introduction, the fundamentals of the quantum mechanics of molecules are presented first that is, the localized side of Fig. 1.1 is examined, basing the discussion on that of Levine (1983), a standard quantum-chemistry text. Details of the calculation of molecular wave functions using the standard Hartree-Fock methods are then discussed, drawing upon Schaefer (1972), Szabo and Ostlund (1989), and Hehre et al. (1986), particularly in the discussion of the agreement between calculated versus experimental properties as a function of the size of the expansion basis set. Improvements on the Hartree-Fock wave function using configuration-interaction (Cl) or many-body perturbation theory (MBPT), evaluation of properties from Hartree-Fock wave functions, and approximate Hartree-Fock methods are then discussed. 

The GAPT charges are also insensitive to basis set improvements beyond the split-valence sets. Table 8 lists the GAPT and Mulliken charges for a variety of small molecules using the STO-3G, 4-31G, and 6-31G " basis sets. Only very small difference between the 4-3IG and 6-3IG" GAPT charges are evident. 

In this study, the validity of quanmm chemical methods and basis sets has been assessed for sulfonamide geometrical parameters. Comparison with experimental data showed that including f-type polarization functions into basis sets improved the geometry optimizations at the Hartree-Fock, MP2, and B3LYP levels. Semiem-pirical methods did not reproduce the experimental sulfonamide bond lengths, bond angles, and torsion angles. 

---