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Notebook[{
Cell["\<\
(***************************************************
Hueckel: Calculates Hueckel orbital energies (ex-
pressed in terms of the energy-related variable x)
and the corresponding orbital matrix C. Enter the 
Fock matrix F simplified and transformed as shown 
in Exercise 4-21 beginning on the third line of code.

Date: 9/27/96 and 2/9/97.

*****************************************************)

(*Turn off annoying error messages.*)

Off[General::spell, General::spell1]

(*Remove all entities created earlier.*)

ClearAll[\"Global`*\"]

(*Enter the Hueckel version of the Fock matrix here.
Data given are for the BUTADIENE Pi system.*)

Forbital = {{0, -1., 0,  0},
\t\t   {-1., 0, -1., 0},
\t\t\t{0, -1., 0, -1.},
\t\t\t{0,  0, -1., 0}};

(*Calculate the dimension n of Forbital.*)

n = Length[Forbital];

(*Calculate the energy eigenvalues and eigenvectors
from the Hueckel Fock matrix.  Separate the eigen-
values in Eorbital and the eigenvectors in Corbital.
The trick for sorting the eigenvalues and eigen-
vectors is suggested by J.M. Feagin, \"Quantum Mech-
anics Methods with Mathematica,\" Springer-Verlag,
1994, p. 112.*)

{Eorbital, Corbital} =
\t\tEigensystem[Forbital] // Chop // Transpose //
\t\t\t\t\t\t\t\t      Sort // Transpose;

(*Transpose Corbital so the orbital coefficients are
listed in columns rather than rows, as is customary.*)

Corbital = Transpose[Corbital];

(*Print the orbital energies Ei.*)

Print[\" \"]
Print[\"Orbital energies (eigenvalues of x):\"];
Print[TableForm[N[Eorbital, 3],
\t\tTableHeadings -> {Range[n], {\" x\"}}]];

(*Print the orbital matrix C.*)

Print[\" \"]
Print[\"Orbital matrix:\"]
Print[TableForm[N[Corbital, 3],
\t\tTableSpacing -> {1, 2},
\t\tTableHeadings -> {None, Range[n]},
\t\tTableAlignments -> Center]];
\t\t\t\t   
\
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