Hello Everyone, I understand that sometimes in physics we make a model of some physical system purely for theoretical convenience, as it allows us to calculate certain properties of the system very easily; although, sometimes these models aren't actually what is going on, but it doesn't matter, so long as the model accurately predicts the effects. In the given link http://www.phy.duke.edu/~lee/P53/therm2.pdf , they suppose such a model. Here is what they say: "To be able to count the degrees of freedom we must make a model of the molecule. We will treat the atoms as point masses, and imagine that the bonds between atoms are like stiff springs connecting these masses." How accurate is this model, and when does it begin to break down?