We investigate the effect of an applied field the internal molecular field
produced by donor-acceptor groups, as well that of an effective field by th
e environment, on the electronic properties and geometric structure of subs
tituted polyenes of finite length. We use analytically tractable methods to
study (a) thr effect of static field on the bond length alternation (BLA)
of finite polyene rings and (b) the effect of static field on even and odd
open chains. We find that the field drives the rings to lower BLA and that
odd chains in a static field can show reversal of BLA through the equal bon
d length limit.