I've read an article about computational chemistry in which the authors were performing a geometry optimization. For this purpose, they firstly optimized the geometry at HF/6-311++G(d,p) level with full relaxation on the potentiel energy surface (what does that mean exactly ? Can we extract this information from the basis set or from the HF method in general?). And after that, they re-optimized at B3LYP level (with the same basis set). Why perform HF and then B3LYP ? Thank you for your help !