Ab initio calculations of adiabatic and diabatic potential energy surfacesof Cl(P-2)center dot HCl((1)Sigma(+)) van der Waals complex

Adiabatic and diabatic potential energy surfaces for the Cl(P-2) atom inter acting with the HCl molecule are calculated at the restricted coupled clust er singles, doubles, and noniterative triples [RCCSD(T)] level of theory an d with the extended augmented correlation-consistent polarized valence-trip le-zeta basis set supplemented with bond functions. An approximate counterp oise correction is applied to evaluate interaction energy of three adiabati c states: 1 (2)A', 2 (2)A', and the 1 (2)A'. Next, the adiabats are transfo rmed to four diabats. The mixing angle of the adiabatic-diabatic transforma tion is determined from the transition matrix elements of the angular momen tum operator L-y calculated using the adiabatic multireference configuratio n interaction wave functions. At the RCCSD(T) level of theory the global mi nimum of the 1 (2)A(') surface occurs for the T-shaped geometry at theta =9 0 degrees and R=3.0 Angstrom with the well depth D-e=586 cm(-1). There is a lso a local minimum at the collinear geometry Cl . . .H-Cl. The global mini mum 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 exhib its two collinear minima, and the global one is for the Cl . . .H-Cl arrang ement 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.