QUANTUM COMPUTATIONAL STUDIES ON PORPHYCENE - Theoretical and Computational Research in the 21st Century - page 170

on the basis of HMO calculations with those obtained from Austin model
1(AM1) method [15, 16] of the corresponding compound.
The bond orders, bond lengths, and π charge densities of the corre-
sponding molecules are calculated on the basis of Huckel molecular or-
bital theory [17, 18] and using AcuraHuckel Version 1.2 software [43].
We have also applied the best semiempirical calculation procedure
Austin Model 1(AM1) [15, 16] for the study of the electronic structure
and chemical reactivity of the chemical systems, pyrrol and porphycene.
The AM1 calculations are carried out using ArgusLab4.0 software [44].
All the structures considered in this study were initially taken as planar,
and the geometry optimizations were done using Hartree-Fock self-con-
sistence fi eld (HF-SCF) method and a minimal STO-3G basis set is used.
The optimized structure and the HOMO and LUMO charge densities are
used in the fi gure to understand the most stable (less repulsion) confi gura-
tion and electronic distribution of the molecules.
9.3.1 HMO CALCULATIONS
In the pioneering days of quantum chemistry, Erich Hückel put forward
an approximate method for solving the Schrödinger equation of benzene,
which was equally suitable for other molecules containing conjugated
π-electrons. This approach is nowadays known under the name HMO the-
ory. The HMO model was originally introduced to permit qualitative study
of the π-electron systems in planar, conjugated hydrocarbon molecules. It
is thus most appropriate for molecules such as benzene or butadiene, but
the approach and concepts have wider applicability. Solving the HMO sec-
ular equation for complex molecules can become very difficult by hand.
However, we enlist the help of the computer program AcuraHuckel
Version 1.2 [43] to solve the secular equation of pyrrol and porphycene.
For pyrrol, the determinant is