A new algorithm, coupling mechanic molecular sigma potentials with a previously derived nonempirical Heisenberg Hamiltonian for the pi electrons, is presented. It makes possible a direct and efficient geometry optimization of any neutral state (in the sense of valence bond theory) for large-size conjugated hydrocarbons with reduced computational effort. The method is tested on planar and twisted conformers of various conjugated systems. The predicted geometries and energy barriers are in good agreement with experimental data and refined ab initio calculations. The potential energy surfaces of the stilbene molecule in its lowest S-0 and T-1 states are studied as a function of the torsional angles around the ethylenic C=C and the C-phenyl bonds.