Computational chemistry Pariser-Pople-Parr method

By the middle of the 1960s, computers had improved, but they were still incredibly slow by today s standards. Therefore, the quantum chemistry of the time was dominated by semiempirical methods, such as the Pople-Pariser-Parr Method, the Wolfsberg-Helmholtz Method, and the Extended Hiickel Method. 

The 1998 Nobel prize in chemistry was shared by Walter Kohn, one of the developers of density-functional theory, and John A. Pople, one of the developers of the Gaussian series of programs and widely used Gaussian basis sets, the Pariser-Parr-Pople method, the CNDO and INDO methods, and one of the first to apply the MP and CC methods to molecular calculations. The Nobel committee noted that computational quantum chemistry is revolutionising the whole of chemistry. ... 

Today we know that the HF method gives a very precise description of the electronic structure for most closed-shell molecules in their ground electronic state. The molecular structure and physical properties can be computed with only small errors. The electron density is well described. The HF wave function is also used as a reference in treatments of electron correlation, such as perturbation theory (MP2), configuration interaction (Cl), coupled-cluster (CC) theory, etc. Many semi-empirical procedures, such as CNDO, INDO, the Pariser-Parr-Pople method for rr-eleetron systems, ete. are based on the HF method. Density functional theory (DFT) can be considered as HF theory that includes a semiempirical estimate of the correlation error. The HF theory is the basie building block in modern quantum chemistry, and the basic entity in HF theory is the moleeular orbital. 

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