4162 Notes molecular orbitals (MO), in particular of the HO (high-est occupied) MO and the LU (lowest unoccupied) MO, have been made in relation to the favorable path of chemical reactions, 4-8 and the nature of HOMO-LUMO interaction has been understood in connection with the change of molecular geometry along the reaction co-ordinate. 9· 10 The conservation of orbital symmetry along the reaction coordinate has been discussed in connection with the stereoselection rule. 3· 11 In this connection, it is desirable toformulate the “reaction coordinate.” Although in actual sense this can obviously never be a simple concept, a conventional definition would be helpful. We tentatively define the reaction coordinate as a curve passing through the initial and the transition points and orthogonal to en-ergy equipotential contour surface. A reaction co-ordinate, defined as such, may be called “intrinsic re-action coordinate.”