In
computational chemistry
Computational chemistry is a branch of chemistry that uses computer simulations to assist in solving chemical problems. It uses methods of theoretical chemistry incorporated into computer programs to calculate the structures and properties of mol ...
, NDDO (neglect of diatomic differential overlap) is a formalism that was first introduced by
John Pople
Sir John Anthony Pople (31 October 1925 – 15 March 2004) was a British theoretical chemist who was awarded the Nobel Prize in Chemistry with Walter Kohn in 1998 for his development of computational methods in quantum chemistry.
Ear ...
; it is the basis for most
semiempirical methods. While
INDO added all one-centre two electron integrals to the
CNDO/2 formalism, NDDO adds all two centre integrals for repulsion between a charge distribution on one centre and a charge distribution on another centre.
[J. Pople and D. Beveridge, ''Approximate Molecular Orbital Theory'', McGraw-Hill, 1970] Otherwise, the
zero-differential overlap approximation is used. The common software program is
MOPAC (Molecular Orbital PACkage).
In the NDDO method, the
overlap matrix ''S'' is replaced by the unit matrix. This allows the
Hartree–Fock secular equation to be replaced with a simpler equation,
. The two-electron integrals from the NDDO approximation can either be one-, two-, three- or four-centered.
The one- and two-centered integrals are evaluated approximately or parameterized based on the experimental data, while the three- and four-centered integrals vanish. Usually, only the valence electrons are treated quantum mechanically, while the role of the
core electrons is to reduce the
nuclear charge
In atomic physics, the effective nuclear charge of an electron in a multi-electron atom or ion is the number of elementary charges (e) an electron experiences by the nucleus. It is denoted by ''Z''eff. The term "effective" is used because the shi ...
. Semiempirical calculations are usually carried out in a minimal
basis set.
See also
*
MNDO MNDO, or Modified Neglect of Diatomic Overlap is a semi-empirical method for the quantum calculation of molecular electronic structure in computational chemistry. It is based on the Neglect of Diatomic Differential Overlap integral approximation. ...
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AM1
*
PM3
*
SAM1 SAM1, or "Semiempirical ab initio Model 1", is a semiempirical quantum chemistry
Quantum chemistry, also called molecular quantum mechanics, is a branch of physical chemistry focused on the application of quantum mechanics to chemical systems, pa ...
*
RM1
*
MOPAC
References
Semiempirical quantum chemistry methods
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