Multistate active spaces from local CAS-SCF molecular orbitals: the photodissociation of HFCO as an example

A recently developed algorithm to generate localized molecular orbitals (LMO) is applied to the study of excited states along a photodissociation process. The LMOs allow for the selection of a consistent complete active space (CAS) for the simultaneous description of all the electronic states involved in a multistate process on the basis of simple chemical criteria. The local nature of the orbitals is used to label them in a unique way that does not depend on the molecular geometry. The selection of the electronic configurations of interest for the set of target states on only the basis of the dominant excitations required by the simplest configuration interaction (CI) descriptions for both ground and excited states is fairly simplified. The following of the changes in the nature of the states along cuts in the set of potential energy surfaces (PES) is also simpler. The C--F bond breaking and the states involved in the photodissociation of the HFCO system, together with another geometrical path along the PES of the same previous states, are studied by way of examples in this work. The local nature of the MOs warrants that this small system is representative enough to show the advantages of the procedure and that its application to larger systems (R-FCO systems) would be straightforward.