Molecular geometries from spin Hamiltonian calculations through simultaneous optimization of geometry and wave function

The recently developed spin Hamiltonian approach to conjugated-electron molecules is reexamined. A simultaneous optimization of the geometry and wave functions, achieved by the use of the conjugate gradient method, facilitates calculations of the molecular geometries in the ground and excited electronic states. The computation time increases approximately linearly with the number of basis functions, making calculations for molecules having up to 18 carbon atoms (48 620 basis functions) readily available. Geometries of several benzenoid hydrocarbons are optimized and the results are discussed.