General Semiempirical MO and DFT Methods

The PPP method does not attempt to explicitly specify integrals theoretically. Rather, the integrals // I and are calculated from approximate semiempirical formulas, some of which contain empirical parameters. For example, when the AOs fr and fs are on atoms R and S that are bonded to each other, /f may be taken as k fr fs), where the valne of the empirical parameter A is chosen so that the predictions of the theory give good agreement with experiment the overlap integral fr fs) is calcnlated from the STOs fr and fs, and not taken as zero as in (17.62). When the two different atoms R and S are not bonded to each other, is taken as zero. (Several versions of the PPP theory 

To do a PPP calculation, one starts with the HMO coefficients as an initial gness for the Csi s, calculates the initial density matrix elements calcnlates the initial matrix elements, solves the equations (17.59) for Tr-electron orbital energies s, and an improved set of coefficients c , calculates improved values, and so on nntil convergence is reached. To improve the results. Cl of the tt electrons may be inclnded. 

The PPP method gives a good acconnt of the electronic spectra of many, bnt not aU, aromatic hydrocarbons. For more on the PPP method, see Parr, Chapter 111 Murrell and Harget, Chapter 2 Offenhartz, Chapter 11. 

The PPP method is not used nowadays and has been superseded by more general semiempirical methods (Section 17.4). However, the PPP method is of historical importance, since many of the PPP approximations used to evaluate integrals are used in current semiempirical theories. 

The HMO and PPP methods apply only to planar conjugated molecules and treat only the 77 electrons. The semiempirical MO methods discussed in this section apply to all molecules and treat all the valence electrons. 

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AMI AMBER A Program for Simulation of Biological and Organic Molecules CHARMM The Energy Function and Its Parameterization Combined Quantum Mechanics and Molecular Mechanics Approaches to Chemical and Biochemical Reactivity Density Functional Theory (DFT), Hartree-Fock (HF), and the Self-consistent Field Divide and Conquer for Semiempirical MO Methods Electrostatic Catalysis Force Fields A General Discussion Force Fields CFF GROMOS Force Field Hybrid Methods Hybrid Quantum Mechanical/Molecular Mechanical (QM/MM) Methods Mixed Quantum-Classical Methods MNDO MNDO/d Molecular Dynamics Techniques and Applications to Proteins OPLS Force Fields Parameterization of Semiempirical MO Methods PM3 Protein Force Fields Quantum Mechanical/Molecular Mechanical (QM/MM) Coupled Potentials Quantum Mecha-nics/Molecular Mechanics (QM/MM) SINDOI Parameterization and Application. 

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