Valence bond diatomics-in-molecules (DIM) treatment of collinear interactions of Group IIa and IIb metal atoms with hydrogen halides: Application to the CaHCl system

A recently developed formulation of the semiempirical valence bond diatomics-in-molecules (DIM) method is applied to the study of collinear MHX systems, where M is a Group IIa or IIb metal atom and X is a halogen atom. In particular, ground and excited state potential energy surfaces for reactions involving the three collinear arrangements of Ca, H, and Cl are considered. Explicit construction of the DIM Hamiltonian matrix is discussed. The resulting ground state surface for the collinear reaction Ca+ClH→CaCl+H, which exhibits a barrier of 34 kcal/mole at a rather stretched nuclear geometry, is shown to be in qualitative agreement with an ab initio surface for the analogous collinear Be+FH system. In addition, a description of the ''harpooning'' mechanism in the collinear reactions CaH+Cl→Ca+HCl and Cl+CaH→ClCa+H arises naturally from the calculations. Reaction pathways and reactant-to-product correlation diagrams are also discussed. A simple estimation of the sensitivity of the most prominent features of the calculated potential energy surfaces to the input diatomic fragment data demonstrates that these features cannot be attributed to errors made in those data