Ab initio calculations of adiabatic and diabatic potential energy surfaces of Cl(2P)⋯HCl(1Σ+) van der Waals complex

Adiabatic and diabatic potential energy surfaces for the Cl(2P) atom interacting with the HCl molecule are calculated at the restricted coupled cluster singles, doubles, and noniterative triples [RCCSD(T)] level of theory and with the extended augmented correlation-consistent polarized valence-triple-zeta basis set supplemented with bond functions. An approximate counterpoise correction is applied to evaluate interaction energy of three adiabatic states: 1 2A′, 2 2A′, and the 1 2A″. Next, the adiabats are transformed to four diabats. The mixing angle of the adiabatic–diabatic transformation is determined from the transition matrix elements of the angular momentum operator L̂y calculated using the adiabatic multireference configuration interaction wave functions. At the RCCSD(T) level of theory the global minimum of the 1 2A′ surface occurs for the T-shaped geometry at θ=90° and R=3.0 Å with the well depth De=586 cm−1. There is also a local minimum at the collinear geometry Cl⋯H–Cl. The global minimum of 2 2A′ occurs for the collinear arrangement H–Cl⋯Cl at R=3.75 Å and with the well depth De=126 cm−1. The 1 2A″ state exhibits two collinear minima, and the global one is for the Cl⋯H–Cl arrangement at R=4.0 Å with the well depth De=429 cm−1. State crossings were also detected: one crossing between the 2Σ+ and 2Π states near R=3.0 Å for the Cl⋯H–Cl form, and two others for the H–Cl⋯Cl form, around R=3.4 Å and R=6.0 Å.