Density functional calculations within the Car-Parrinello framework ar
e used to obtain self-consistently the electronic and the geometrical
structures of polythiophene derivatives. The relation between the elec
tronic states and quinoid vs aromatic structures is studied on a serie
s of substituted polythiophenes (PTh) and polyisothianaphthenes (PITN)
, Excellent agreement is found with experimental data, where these are
available. Some shortcomings of semiempirical methods in describing p
i bonds with heteroatoms are revealed, A unique minimal energy structu
re is obtained for each of the polymers; no other local minima are fou
nd, The general rule is that a polymer adapts the structure that leads
to the largest possible band gap, which for PTh-like polymers is the
aromatic structure and for PITN-like polymers is the quinoid structure
. Substitutions do not change the basic geometry and electronic struct
ure of either PTh or PITN but can alter the band gap by several tenths
of an electronvolt.