Mathematical "non-analyticities" may occur in potential energy curves caicuiated by way of single configuration molecular orbital (MO) theory (at any level of sophistication), for processes in which the MO configuration is not conserved [I ] . Ways of avoiding these "non-analyticities" have been suggested [l] *. Tke processes themselves have been classified as "thermally forbidden, photochemically allowed" by Woodward and Hoffmann [2] and by others as "electronically forbidden" [3]. In situations with symme-try> they are characterized by a change in the lowest energy electron configuration, i.e., a change in orbital occupancy, during the course of reaction. Here we .examine the adequacy of three different LCAO MO methods for the construction of reliable potential energy surfaces for such processes. These are (i) Non-self-consistent field (non-SCF) LCAO single configuration methods, e.g., the extended Hiickel method [4] and non-SCF anti-symmetrized MO methods. (ii) Single configuration LCAO SCF methods [S, IO] ,
especially the restricted (closed shell) Roothaan-Hartree-Fock SCF method which is csed widely in many apprqximate and semi-empirical theories
[ii].
(iii) The extension [6-10,121 of these methods to.
* On the whok the discussion in this-earlier paer,assumes an LCAO form for the MO's; it is in,Ws amtext that both 03 &pied a.rid unocfupied orbita@ are obt&ed.