Force in SCF theories. Test of the new method

Abstract

A new force method reported previously, which may be called the (r, r′) method, is tested for several di- and triatomic molecules, CO, N2, LiF, H2O, and HNO, with the use of several basis sets. It is shown that by the addition of the first derivatives of the basis set, the Hellmann–Feynman (H–F) theorem is essentially satisfied. The present force method is shown to be useful for calculations of equilibrium geometries and force constants. The basis set of double zeta accuracy such as 4-31G, 6-31G, and [3s2p/2s] sets are well suited for the present method. The STO-3G set seems to be less suitable. Further it is shown that by the present method the other one-electron properties are improved at the same time. We have discussed the features of the present method as compared with the energy gradient method.