Abstract: The STO-xGfixed single ξ basis functions of variable expansion length with the regression values of ζ or any values belong 10 the ζ interval of confidence level of 99.7% are presented. Comparison of the STO-xG (for example STO-6.3G) fixed single ζ mini, basis set with the appropriate STO-KG (for instance STO-4G)basis set in SCFcalculations illustrates that the former not only takes less CPUtime and provides better results, but also is practical and systematical. In addition, the results of this paper express that the proportion of kinds of basis functions of different angular momenta in the extented part of extented basis set should not overlooked, and the ζ interval of confidence level of 99.7% is valuable in considering the regions for optimizing ζ values.