instead of generating one active space for a given excitation, we generate several active spaces and choose between them variationally with tPBE. We call the new active space selection scheme "λAPC-tPBE", and it is composed of four steps: (i) orbital construction; (ii) APC entropy calculation; (iii) orbital selection with APC entropies; and (iv) variational selection with tPBE. In the following we explain these components step-by-step.