  Modeling chemical reactions with quantum chemical methods is challenging when
the electronic structure varies significantly throughout the reaction, as well
as when electronic excited states are involved. Multireference methods such as
complete active space self-consistent field (CASSCF) can handle these
multiconfigurational situations. However, even if the size of needed active
space is affordable, in many cases the active space does not change
consistently from reactant to product, causing discontinuities in the potential
energy surface. The localized active space SCF (LASSCF) is a cheaper
alternative to CASSCF for strongly correlated systems with weakly correlated
fragments. The method is used for the first time to study a chemical reaction,
namely the bond dissociation of a mono-, di-, and triphenylsulfonium cation.
LASSCF calculations generate smooth potential energy scans more easily than the
corresponding, more computationally expensive, CASSCF calculations, while
predicting similar bond dissociation energies. Our calculations suggest a
homolytic bond cleavage for di- and triphenylsulfonium, and a heterolytic
pathway for monophenylsulfonium.
