A new force method reported previously, which may be called the (r,rprime;) method, is tested for several dihyphen; 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 Hellmannndash;Feynman (Hndash;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 4hyphen;31G, 6hyphen;31G, and lsqb;3s2p/2srsqb; sets are well suited for the present method. The STOhyphen;3G set seems to be less suitable. Further it is shown that by the present method the other onehyphen;electron properties are improved at the same time. We have discussed the features of the present method as compared with the energy gradient method.
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