COMPUTATION OF CURVE-CROSSING DIAGRAMS BY APPROXIMATE VALENCE-BOND METHOD

Avoided crossing diagram parameters for the radical exchange reaction and the concerted exchange of two and three bonds are computed by usin g the approximated valence bond method, which is a nonorthogonal confi guration interaction (CI) semiempirical method among the valence bond configuration functions. Here, each valence bond configuration functio n is a spin-adapted combination of Slater determinants constructed fro m the Heitler-London or Coulson-Fischer hybrid orbitals. Atomic orbita ls integrals are evaluated using semiempirical philosophy, and these p rovide considerable saving of computer time compared with the most sta ndard ab initio multistructure valence bond methods. The results indic ate that the approximate valence bond method is capable of yielding re asonable results for the avoided crossing diagram parameters. These re sults also indicate that the diagram gap (G) is the decisive factor fo r the stability of symmetric clusters, X(n), although no clear correla tion between the gap G and the geometric distortion is found for diffe rent values of n. (C) 1996 John Wiley & Sons, Inc.