Zero Differential Overlap and the CNDO Methods
- filipemmenezes
- May 28, 2020
- 1 min read
The next advent in semiempirical methods comes with the introduction of the zero differential overlap approximation, which states that the product of two orbitals and centered, respectively, in atoms and is non-zero if and . Because of this, CNDO methods are already iterative in nature. The approximation is applied to spherically symmetric operators and was first employed to -systems in the Pariser-Parr-Pople method (Lykos, P.; Parr, R. G.; J. Chem. Phys., 24, 1166, 1956; Pariser, R.; Parr, R. G.; J. Chem. Phys.; 21, 466, 1953; Pariser, R.; Parr, R. G.; J. Chem. Phys., 21, 767, 1953; Pople, J. A.; Trans. Far. Soc., 49, 1375, 1953). The first successful extension to – and -systems was due to Pople and coworkers with the first complete neglect of differential overlap (CNDO/1) method.
CNDO/1 suffers from many faults due to the underlying approximations, and the method was later modified by the same group, yielding what is now currently known as CNDO/2.
Other variants of the method were also developed in other groups. Examples are CNDO/BW and CNDO/BG.
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