I'm learning about how Gaussian works and I'm reading about potential energy surfaces which Gaussian uses to calculate very properties of a molecule. Right now I'm reading about optimisations, I understand the concept that to optimise the structure of a molecule, it needs to find the global minimum (I'm still not sure what local minima represent) but there is a lot of terminology I don't understand. Firstly what do they mean when they say "convergance criteria"? They say that when convergance is reached, the structure is optimised but what does this mean? Secondly they say that a minimum is reached when the force is 0. Is the force just the first derivative of the energy? That would make sense. They also mentioned force constants a few times, what does this mean in the context of potential energy surfaces?