Restricted Hartree-Fock water

Fig. 1 Diagonal elements of the density response function for water in the molecular plane. Coordinates are expressed in atomic units. The wavefunction is restricted Hartree-Fock with the 6-31G basis set. The figure shows raw values of the response function. These must be multiplied by 1.0012 x 10 (twice the inverse square of the volume element) to get the values in atomic units...

A restricted Hartree-Fock study of formation of Schiff base (A-[(Z)-furan-2-ylmethylidene]-4-methoxyaniline) from aromatic amine and furaldehyde has revealed that an auxiliary water molecule enables proton transfer in the carbinolamine-forming... 

In cases where one is restricted to smaller basis sets by computational limitations, a double-C set which has been augmented by a single (well chosen) set of polarization functions offers an attractive alternative. Many of the above properties of H-bonded systems can be computed to excellent accuracy. It was shown above that a basis set containing only 50 functions was able to approach within 0.1 kcal/mol of the Hartree-Fock limit for the interaction energy in the water dimer. Moreover, a basis set of this type is sufficiently flexible to permit meaningful inclusion of electron correlation effects. Again, application of counterpoise correction is advised whenever possible. 

Figure 4. Double bond dissociation of the water molecule using the perfect pairing (PP), imperfect pairing (IP) and restricted pairing (GVB-RCC) local correlation models, compared to full configuration interaction (FCI) and Hartree-Fock theory in a minimal (STO-3G) basis.

When the Hartree-Fock description is reasonably accurate - as for the stable water molecule -the restricted CCSD model appears to provide a satisfactory representation of the FCI wave function. Although less accurate than CCSDT, it represents a useful compromise between accuracy and cost and can be routinely applied to relatively large molecular systems in a black-box manner. The excellent performance of the CCSDT model is quite remarkable but its cost is so high that it is applicable only to small systems. For most purposes, however, the CCSDT model may be replaced by the CCSD(T) model at little or no loss of accuracy, making it possible to carry out highly accurate calculations on systems containing up to ten atoms. 

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