SPIN–CIPSI calculations on H2CO

Abstract
The efficiency of configuration interaction by perturbation with spin eigenfunctions is discussed in the case of ground and excited states of formaldehyde in its ground state equilibrium conformation. Two extensive configuration interactions using for every excited state more than 40 000 configurations eigenfunctions of spin, including six open shells, is carried out using the s p i n c i p program, spin‐symmetry‐adapted generalization of c i p s i.