A partial approximation in calculations of secondhyphen;order perturbation energies

摘要

In calculations of molecular problems, only a part of excited electronic states corresponding to low excitation energies can be obtained with numerically reliable accuracy. Then, in the series of the secondhyphen;order perturbation energy, some of the intermediate excited states corresponding to low excitation energies are retained explicitly but others are eliminated by use of a mean excitation energy and completeness relations. The appropriate value of the mean excitation energy is discussed for the case of the proton spinndash;spin coupling in CH4.