ABSTRACT
The (2)A' and (2)A'' adiabatic potential energy surfaces for the He-NCO(X(2)Pi) van der Waals system are obtained by the partially spin-restricted coupled cluster method with single, double, and noniterated triple excitations (RCCSD(T)). The ab initio potentials are fit to analytical expressions, and scattering and bound state calculations are performed for a rigid NCO((2)Pi) radical. Rotational constants of the complex are reported. The scattering calculations of integral and differential cross sections are performed using both the fully quantum close-coupling (CC) and coupled-states (CS) methods. The collision energies have the values taken from the experiment of Macdonald and Liu (J. Chem. Phys. 1992, 97, 978). The excellent agreement between theoretical and experimental scattering results attests good quality of the ab initio potential.
ABSTRACT
This study reports an ab initio characterization of a prereactive van der Waals complex between an open-shell atom Br((2)P) and a closed shell molecule HBr. The three adiabatic potential surfaces 1 (2)A('), 2 (2)A('), and 1 (2)A("), which result from the splitting of degenerate P state of Br are obtained from coupled cluster calculations. The coupling between same-symmetry states is calculated by multireference configuration-interaction method. A transformation to a diabatic representation and inclusion of the spin-orbit coupling effects on the interactions are also discussed. Bound states are calculated using an adiabatic bender model. The global minimum on the lowest adiabatic potential surface corresponds to a T-shaped geometry and has a well depth of D(e)=762.5 cm(-1) at R(e)=3.22 A. A secondary minimum occurs for a hydrogen-bonded geometry with D(e)=445.3 cm(-1) at R(e)=4.24 A. Upon inclusion of spin-orbit coupling the hydrogen-bonded minimum remains at the same depth, but the T-shaped minimum washes out to less than half of its spin-free value. The lowest bound state is localized in the linear minimum. The spin-orbit coupling plays a very important role in shaping of the potential energy surfaces of Br-HBr.
