A. The Q P philosophy P is a standalone easy-to-use library for developers. e main goals of P are to i) facilitate the development of new quantum chemistry methods, ii) minimize the dependency on external programs/libraries, and iii) encourage the collaborative and educative work through human readable programs. erefore, from TABLE . Zeroth-order energy E (0) , second-order perturbative correction E and its renormalized version ZE (in hartree) of CN3 for increasingly large wave functions. e excitation energy ∆E (in eV) is the energy di erence between the ground state (GS) and the excited state (ES). e statistical error, corresponding to one standard deviation, is reported in parenthesis.