Calculations of SiO₂ static-fatigue strength are used to show that the strength-degradation behavior of the material under the influence of the ambient atmosphere can be analyzed using an interatomic potential which is based on the Tersoff potential but extended to take charge transfer effects into account. The force-elongation curves of the Si-O interatomic bonds of the SiO₂ are calculated with and without H₂O in the atmosphere. Based on these curves, crack propagation behavior is analyzed, and calculated results are shown to correspond well with experimental results. Moreover, the calculated values of the strength decrease caused by the H₂O also show fair agreement with the experimental values.