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(P-2) 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 (2)A', 2 (2)A', and the 1 (2)A'. 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 L-y calculated using the adiabatic multireference configuration interaction wave functions. At the RCCSD(T) level of theory the global minimum of the 1 (2)A(') surface occurs for the T-shaped geometry at theta =90 degrees and R=3.0 Angstrom with the well depth D-e=586 cm(-1). There is also a local minimum at the collinear geometry Cl . . .H-Cl. The global minimum of 2 (2)A(') occurs for the collinear arrangement H-Cl . . . Cl at R=3.75 Angstrom and with the well depth D-e=126 cm(-1). The 1 (2)A" state exhibits two collinear minima, and the global one is for the Cl . . .H-Cl arrangement at R=4.0 Angstrom with the well depth D-e=429 cm(-1). State crossings were also detected: one crossing between the (2)Sigma (+) and (2)Pi states near R=3.0 Angstrom for the Cl . . .H-Cl form, and two others for the H-Cl . . . Cl form, around R=3.4 Angstrom and R=6.0 Angstrom. (C) 2001 American Institute of Physics.