||Preview - Screen
The main screen allows the user to draw the molecule they want to calculate. When the atom button is pressed
the user can draw atoms on the screen of the kind selected from the dropdown box (Carbon in the example). Single
and double bonds can be drawn between atoms and the user can delete bonds or atoms when making mistakes.
In the drawing the kind of atom can be changed to one of the other available atoms through the menu which popups
when clicking the right mouse button.
From the toolbar the key functions can be accessed: Draw the molecule, choose atoms to draw, build the Hülckel
matrix, diagonalise it and view the results in a level diagram, a MO diagram or by looking at the bare numbers.
In this diagram one set of results are shown from the Hückel calculation; the eigenvalues. These are the
energies of the electrons which were taken into account in the calculation. These eigenvalues are shown as levels
(verical lines) in the drawing. The closed circles denote electrons which occupy the levels. Open circles indicate
how many electrons can still be hold by a certain level.
In the options menu you can enhance the view of the level diagram. You can hide the electrons, show energy values
at each level and show an indicator of the multiplicity for each level. The options menu also allows printing the
The MO diagram is another result from a Hückel calculation. A MO (Molecular Orbital) defines the position
of an molecular electron. In the MO diagram, circles are drawn around the atoms. The size of a circle denotes the
possibility of finding the molecular electron at that site. So if the circle drawn is small then, on average, the
electron spends little time on that atom.
The coefficients making up an MO can be positive (blue) or negative (gray). The number of nodes (points were
positive and negative coefficients are neighbours) is related to the energy. You'll find the MO with the least
nodes the lowest in energy while the MO with the most nodes to be the highest in energy.
Important to know is the occupied MO which is highest in energy. This MO is most important for most chemical
properties. To find out which MO is the highest in energy you have to step through the MOs and observe the occupation
number shown on the screen. MOs are filled from the first MO upwards.