CNDO is the short cut for Complete Neglect of Differential Overlap. It is both 1) a method for learning key facts about molecular orbitals as well as 2) the name of a particular set of computer programs. CNDO is based on quantum chemistry. It is one of the first semi-empirical quantum chemistry methods. It uses two approximations:
- core approximation - only the outer valence electrons are explicitly included.
- zero-differential overlap
CNDO builds a mathematical model of a molecule. CNDO starts with the position of the positively charged nuclei of the atoms and the negatively charged electron clouds surrounding them. Quantum chemistry tells the model the probability distribution of electron locations based on atomic orbitals. From this information, CNDO computes data about the resulting molecular orbitals.
Background[change | change source]
The Extended Huckel method was an earlier method for calculating molecular orbitals and the electronic energy. It explicitly ignores the mathematical equations that show that electrons repel each other ("electron-electron repulsion terms"). CNDO/1 and CNDO/2 were developed from the Extended Huckel method by explicitly including the electron-electron repulsion terms. But CNDO leaves out many of repulsion terms, approximates some of others, and fits others to experimental data from spectroscopy.
How it works[change | change source]
Quantum mechanics provides equations based on the Hartree-Fock method and the Roothaan equations that CNDO uses to model atoms and their locations. These equations are solved over and over (iteratively) to the point where the results do not vary significantly between two repetitions. CNDO does not involve knowledge about chemical bonds but instead uses knowledge about quantum wavefunctions.
Chemists find that CNDO gives results that match experimental data about partial atomic charges and dipole moments. CNDO calculates total energy and binding energy. CNDO computer software reports the Eigenvalues for calculating the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).
Related pages[change | change source]
References[change | change source]
- J. Pople and D. Beveridge, Approximate Molecular Orbital Theory, McGraw-Hill, 1970.
- J. A. Pople, D. P. Santry and G. A. Segal, Journal of Chemical Physics, 43, S129, (1965)
- J. A. Pople and G. A. Segal, Journal of Chemical Physics, 43, S136, (1965)
- J. Pople and G.A. Segal, Journal of Chemical Physics, 44, 3289 (1966)
- D.P. Santry and G.A. Segal, Journal of Chemical Physics, 47, 158 (1967)