Ab initio treatment of the chemical reaction precursor complex Cl(2P)-HF.: 1.: Three-dimensional diabatic potential energy surfaces

The three adiabatic potential surfaces of the Cl((2)p)-HF complex that con-elate with the 2p ground state of the Cl atom were calculated with the ab initio RCCSD(T) method (partially spin-restricted coupled cluster theory including single and double excitations and perturbative correction for the triples). With the aid of a geometry-dependent diabatic mixing angle, calculated by the complete active space self-consistent field (CASSCF) and multireference configuration-interaction (MRCI) methods, these adiabatic potential surfaces were converted to a set of four distinct diabatic potential surfaces required to define the full 3 x 3 matrix of diabatic potentials. Each of these diabatic potential surfaces was expanded in terms of the appropriate spherical harmonics in the angle theta between the HF bond axis r and the Cl-HF intermolecular axis R. The dependence of the expansion coefficients on the Cl-HF distance R and the HF bond length r(HF) was fit to an analytic form. The strongest binding occurs for the hydrogen-bonded linear Cl-HF geometry, with D-e = 676.5 cm(-1) and R-e = 6.217 a(0) when r(HF) = r(e) = 1.7328 a(0). This binding energy De depends strongly on rHF, with larger rHF causing stronger binding. An important contribution to the binding energy is provided by the interaction between the quadrupole moment of the Cl(P-2) atom and the dipole of HF. In agreement with this electrostatic picture, the ground state of linear Cl-HF is a 2-fold degenerate electronic Pi state. For the linear Cl-FH geometry the states are in opposite order, i.e., the I state is lower in energy than the Pi state. The following paper in this issue describes full three-dimensional computations of the bound states of the Cl-HF complex, based on the ab initio diabatic potentials of this paper.