The Born-Oppenheimer approximation is the heart of molecular sciences. As expressed by Heller, it is "one of those wonderful approximations that even in failure forms the basis for discussion and systematic corrections." A lot of the everyday work of computational chemists is concerned with solving the electronic equation (2). A multitude of methods has been developed. Most adopt a bottom-up approach where one-electron wave functions (atomic orbitals) are combined to form molecular wave functions. The electronic wave function is still raw at this point: the electrons don't know about each other. We must let them adapt and relax in the multielectron environment. We do that using electronic structure methods that recover such electronic correlation 23 so that Eq. ( ) is satisfied (Figure ). The electronic correlation corresponds to the orbitals' adaptation to the multielectronic environment.