The meaning of Spartan images

Electron Density Slices

The Spartan program will calculate the electron density in a space around the nuclei of single atoms or the atoms in a molecule. To be able to view the results of these 3D calculations a 2D plane is placed through the atom or molecule. The electron density field is then superimposed on this plane. This plane is usually placed through a nucleus or along the plane of a bond. Colors are used to represent the intensity of the electron density field. The scale bar on the images is a legend to the meaning of the colors. Blue represents are high electron density and red is low. The units are electrons/au3, but this is not important. Many of the images of single atoms show a large blue area around the nucleus. This does not mean that the electron density is constant in this area, it just means that it is at least as high as the value assigned to blue on the color bar. This can be changed by the user of the program. In all atoms the electron density increases as you approach the nucleus.

In quantum mechanical terms, the electron density is actually the square of the wavefunction. This is the probability of finding the electron. An equivalent approach is to think of the electron's charge. Because the electron's exact position is not defined, we can think of its charge as being nonuniformly distributed in the atom or molecule. So, each point in space will have a partial charge. This "charge density" is another way of describing electron density.

Electron Density Isosurfaces.

Electron density isosurfaces are similar to isobars on a weather map. Isobars are lines connecting points with the equal air pressure. Iso- means equal. Electron density isosurfaces are surfaces with equal electron densities around an atom or molecule. They can be displayed as solid surfaces, transparent surfaces, or mesh surfaces. They are gray unless some other data is superimposed on them. There are two default density values used for these surfaces. The value of 0.08 is referred to by the program as a "density (bond)" surface and displays a surface that identifies the high electron density regions in the molecule primarily responsible for holding nuclei together. This surface indicates where the electrons are concentrated in the molecule. The other default surface has a value of 0.002 and shows the outer edge of the molecule. About 90% of the molecule's electron density is inside this surface. This surface is used to study the interaction of a molecule with other molecules. It is referred to by Spartan as the "density" surface. In other words, the surface is used to study the intramolecular forces and the density surface the intermolecular forces. Together these surfaces show the chemical bonds in a molecule.

Electrostatic Potential Isosurfaces

These surfaces are the same as the electron density isosurfaces, but the property used is electrostatic potential. Electrostatic potential is the force felt by a charge placed around a molecule. The charge will be attracted or repel by a force of a certain strength. Therefore, electrostatic potential has both a magnitude and positive or negative sign. Where the electrostatic potential is negative, there is a surplus of electrons and this part of the molecule has a negative charge. Likewise, where the electrostatic potential is positive, there is a deficit of electrons and this part of the molecule has a positive charge.

Electrostatic Potential Plots

Superimposing electrostatic potential plots on the 0.002 isosurface makes intermolecular forces easier to visualize and predict. These surfaces are generated by placing a charge at a constant distance from the isodensity surface all around the molecule. The strength of the electrostatic attraction or repulsion the charge feels is recorded and color coded on the isodensity surface. Blue is used to represent a strongly positive electrical charge and red a strongly negative electrical charge. The other colors are the values in between.


ME, 1998