Use of the vibronic CI method in accurate calculations of the renner-teller effect

Abstract

Calculations of the Renner-Teller effect in various triatomic systems are carried out employing the vibronic Cl method, according to which a matrix representation of the pertinent global hamiltonian is effected and the corresponding eigenvalues and eigenvectors are obtained by a single diagonalization. Conventional least squares polynomial fitting procedures are employed to represent all kinetic and potential energy terms, thereby greatly simplifying the computation of the required matrix elements. Both the adiabatic (bent) and diabatic (linear) transformations of the electronic wavefunctions are employed and the numerical results are compared with one another as well as with data resulting from earlier accurate treatments of the Renner-Teller effect in which analytical expressions for the various hamiltonian terms are treated by means of numerical procedures. Special attention is given to the effect of considering the bending angle dependence of the non-adiabatic coupling terms explicitly. It is found that the discrepancies between computed eigenvalues resulting from the present variational method and those of the earlier numerical integration procedures are no greater (root mean square values of 1-2 cm-1) than have been noted among the numerical methods themselves. © 1983 Taylor & Francis Group, LLC.