In this Letter the Curl condition related to the nonadiabatic coupling term
s, proved to be fulfilled by Born-Oppenheimer electronic eigenfunctions [Ch
em. Phys. Lett. 35 (1975) 112], is extended to include the points of singul
arity as recently discussed [Chem. Phys. 259 (2000) 123, Appendix B]. The s
tudy, related at this stage to a two-state system, is expected to add insig
ht as to the origin of the non-adiabatic coupling terms. (C) 2001 Published
by Elsevier Science B.V.