On the Location of Conical Intersections in CH 2 BrCl Using MS-CASPT2 Methods Tama ´ s Rozgonyi Institute of Structural Chemistry, Chemical Research Center, Hungarian Academy of Sciences, 1025 Budapest, Pusztaszeri u ´ t 59-67, Hungary Leticia Gonza ´ lez* Institut fu ¨r Chemie und Biochemie, Takustrasse 3, Freie UniVersita ¨t Berlin, 14195 Berlin, Germany ReceiVed: December 9, 2005; In Final Form: June 22, 2006 Multiconfigurational second-order perturbation theory has been employed to calculate two-dimensional potential energy surfaces for the lowest low-lying singlet electronic states of CH 2 BrCl as a function of the two carbon- halogen bonds. The photochemistry of the system is controlled by a nonadiabatic crossing occurring between the A ˜ and B ˜ bands, attributed to the b 1 A′ and c 1 A′ states, which are found almost degenerate and forming a near-degeneracy line of almost equidistant C-Br and C-Cl bonds. A crossing point in the near-degeneracy line is identified as a conical intersection in this reduced two-dimensional space. The positions of the conical intersection located at CASSCF, single-state (SS)-CASPT2, and multistate (MS)-CASPT2 levels of theory are compared, also paying attention to the nonorthogonality problem of perturbative approaches. To validate the presence of the conical intersection versus an avoided crossing, the geometrical phase effect has been checked using the multiconfigurational MS-CASPT2 wave function. 1. Introduction Conical intersections (CoIn) are photochemical funnels which allow radiationless transitions between electronic excited states. 1 They occur when at least two potential energy surfaces (PESs) intersect. At the intersection, the two surfaces are degenerate, and a molecule can cross from one electronic state to another. Because the time scale on which a transition through a CoIn occurs is only of a few tens of femtoseconds (fs), as demon- strated by femtosecond laser technology and time-resolved spectroscopy, CoIns are the fastest way for an electronically excited molecule to relax back to the ground state or to a lower- lying electronic state. Until recently, CoIns were thought to be a curiosity; over the past years, it has been established that CoIns are ubiquitous. The high photostability of our genetic code is attributed to CoIns; light harvesting, vision, and a variety of essential upper atmospheric processes involve CoIns; and plenty of organic and inorganic molecules undergo CoIns upon ultraviolet (UV) irradiation. In all cases, the CoIn plays an important mechanistic role in the spectroscopy, photochemistry, and chemical kinetics of such processes. CoIns can be classified according to different criteria, 1 for instance, the role that symmetry plays in their existence. A CoIn is symmetry-required when the intersection occurs between two degenerate electronic excited states that belong to the same irreducible representation. An example 1 is the Jahn-Teller intersection between the two E states in C 3V symmetry of Na 3 . Opposed to symmetry-required CoIns, accidental intersections are those where symmetry does not play a role. Here, one can distinguish between accidental symmetry allowed and same symmetry CoIns. The former corresponds to intersections that occur in a coordinate subspace where the two electronic states have different symmetry, and hence, they may cross freely. The latter refer to intersections between PESs of the same symmetry. According to the noncrossing rule, 2 CoIns between states of the same symmetry are permitted in a space of dimension N - 2, the so-called seam space, where N is the number of internal degrees of freedom (3N - 6). Thus, in diatomics, only states of different symmetry can cross, and states of same symmetry lead to an avoided crossing. In polyatomics, however, the noncrossing rule fails, in the sense that states of any symmetry are allowed to cross at any point of the N - 2 seam space, as pointed out by Teller 3 in 1969, and largely demonstrated in the last years by many advances in computational photochemistry; see, for instance, refs 4 and 5. The lowest-energy point of the seam space or minimal point of the crossing seam (MXS) is normally referred to as the CoIn (although it is simply the lowest CoIn of the crossing seam). Despite their omnipresence, CoIns (or MXS) are difficult to predict and locate. At a CoIn, the Born-Oppenheimer ap- proximation breaks down, making it nontrivial to compute the properties of the system with standard quantum chemical methods. Several methods have been proposed to search for the degenerate points. 4,6-10 For instance, Haas, Zilberg, and co- workers 10 exploit the use of elementary reaction coordinates and the phase change rule to find a CoIn between the ground and lowest electronically excited states. The geometrical phase effect, proved first by Longuet-Higgings, 11 states that, if the wave function changes sign when adiabatically transported round a loop in the nuclear configuration, then the loop must contain an odd number of CoIns. Consequently, given a loop for which the adiabatic wave function changes sign once, the loop contains a single CoIn. Therefore, one can construct a loop using, for instance, three points in the electronic ground state, for example, two reactants and one product; and by using elementary reaction coordinates, it is possible to find the point of degeneracy where the phase changes only once between two points. 10 We note that this method, to our knowledge, has not been applied to locate CoIns between different electronically excited states. * Corresponding author. E-mail: leti@chemie.fu-berlin.de. 10251 J. Phys. Chem. A 2006, 110, 10251-10259 10.1021/jp057199s CCC: $33.50 © 2006 American Chemical Society Published on Web 08/05/2006