Multiconfigurational second-order
perturbation theory has been em
ployed to calculate two-dimensional
potentialenergy surfaces for the lowest low-lying singlet electronic states of CH
2BrCl as a function of the two carbon-halogen bonds. The
photochemistry of the system is controlled by a nonadiabatic crossing occurring betweenthe à and
![](/images/entities/Btilde.gif)
bands, attributed to the b
1A
' and c
1A
' states, which are found almost degenerate and forming anear-degeneracy line of almost equidistant C-Br and C-Cl bonds. A crossing
point in the near-degeneracyline is identified as a conical intersection in this reduced two-dimensional s
pace. The
positions of the conicalintersection located at CASSCF, single-state (SS)-CASPT2, and multistate (MS)-CASPT2 levels of theoryare com
pared, also
paying attention to the nonorthogonality
problem of
perturbative a
pproaches. To validatethe
presence of the conical intersection versus an avoided crossing, the geometrical
phase effect has beenchecked using the multiconfigurational MS-CASPT2 wave function.