one- and two-electron operators

[ilfreddy]

Is there any good (code-related) reason to keep 1- and 2-el operators separate in the code? As a matter of coding, this distinction seems to me rather arbitrary. I'm starting to code ZORA and I am implementing the ZORA "potential" (1-V/2c^2) and its inverse "kappa". In this first implementation I'm only considering the nuclear potential and therefore the ZORA potential is a 1-el operator. I have also implemented the corresponding kinetic energy operator (p kappa p)/2. As long as V is the nuclear potential all such operators will be 1-el, but as soon as I will add the coulomb and xc pieces, they will become 2-el operators and I will have to move them to the other folder. Does not make much sense to me.