The effect of a mechanistic drop evaporation model on the pressure-temperature transient of a containment under loss-of-coolant accident (LOCA) conditions has been investigated. To implement the model, the traditional two-node lumped parameter (atmosphere and sump) had to be expanded to encompass additional open thermodynamic systems. The calculations were compared against results obtained by a widely employed containment analysis code using the instantaneous evaporation model. The mechanistic drop evaporation model was found to produce higher peak pressures and substantially higher degrees of superheat for a steam line break LOCA. The dependence of pressure in both saturated and superheated air-steam atmospheres was generalized in terms of normalized pressure-energy derivatives. For superheated atmospheres, these derivatives were found to depend on the mode of energy removal. Two idealized energy removal modes were defined (purely condensing and purely noncondensing). The normalized pressure-energy derivatives for these mechanisms were found to differ by a factor of 2 to 3 for the parameter range of interest to containment analysis.