Ab initio quasidiabatic states for the reaction N + CH → NC + H Original Research Article

A procedure for computing a global ab initio quasidiabatic representation is presented. It is numerically tested on the N + CH reaction restricted to collinear configurations. The method involves firstly the computation of a set of adiabatic states with the help of the CASSCF method and state averaged molecular orbitals. Then the adiabatic set (or a subset of it) is transformed to a quasidiabatic set by a unitary transformation, which is build such that it maximises the overlap between the quasidiabatic wavefunctions at successive nuclear configurations. The numerical results show that a large part of the derivative couplings is transformed away. Owing to the fact that the remaining derivative couplings are multiplied by a term proportional to the nuclear velocity, they will be largely dominated by the electronic couplings at low velocity, thus allowing to neglect them. It is shown also how the quasidiabatic representation can be used to detect conical intersections not due to the symmetry.