Introduction

WinPPP is a small quantum chemical (QC) program especially useful for students of organic chemistry to explore structure-property relationships of organic molecules having a conjugated p- electronic framework.
QC calculations are performed on the basis of Huckel Molecular Orbital (HMO) and the Pariser-Parr-Pople (PPP) algorithms. Some new developments made by Zahradnik and Nishimoto have been considered and may be used if desired.

The main goal of WinPPP is to calculate singlet state properties of p-systems, such as the UV-Vis absorption spectra and transition probabilities as well as non-linear optical properties up to the third order (harmonics only).

WinPPP consists of three main parts:

• Molecule preparation
  Three-dimensional structures may be imported from MOPAC, Gaussian, Alchemy, MDL, or SMD type files. Alternatively, two-dimensional molecular structures can be created by an internal editor which is also capable of changing atom types of all imported molecules.
• QC calculations
  Lots of options allow every user to define a specific type of a PPP calculation to fit various requirements, e.g.:
  · Usage of Slater-type 2p orbitals for integral and transition moment computations is fully supported.
  · Refinement of standard PPP parameters due to polarisations within the s-framework as proposed by Zahradnik may be included.
  · Additional 2D geometry optimisations according to the actual bond orders can be performed during the SCF procedure.
  · Transition moments may be calculated by the use of the Dipole Velocity or Dipole Length formulae.
  · For the calculation of the first, second and third molecular polarisabilities at any wavelength, a unique damping factor used in the Sum-Over-States method can be specified.
  · Each result (even the Nabla operators and transition moments between the excited states) can be written to a file during the calculation.
• Visualisation of Results
  Nearly all calculated results can be visualised, e.g.:
  · Molecular orbitals, charge distributions and transition densities.
  · Dipole and transition moments from the ground to excited states.
  · UV-Vis absorption spectra (experimental spectra may be loaded for comparison).
  · Energy levels and complete CI (including exchange and coulomb integrals).