  Free electron approximation for electron states in poly($p$-phenylene
vinylene) and other conjugated systems is developed. It provides simple and
clear analytical expressions for energies of electron states and for wave
functions. The results are in quantitative agreement with experiments. Our
model does not contain fitting parameters. We present two examples of the
developed theory applications: exact calculation of electron energy in magnetic
field, and scattering of electrons in copolymer.
