Semiempirical PRDDO

The PRDDO (partial retention of diatomic differential overlap) method is an attempt to get the optimal ratio of accuracy to CPU time. It has been parameterized for the periodic elements through Br, including the 3rd row transition metals. It was parameterized to reproduce ah initio results. PRDDO has been used primarily for inorganic compounds, organometallics, solid-state calculations, and polymer modeling. This method has seen less use than other methods of similar accuracy mostly due to the fact that it has not been incorporated into the most widely used semiempirical software. 

In general, CNDO/2, INDO, and PRDDO mimic ab initio results by artful and compensating approximations and semiempirical parameters, and they yield reasonable dipole moments and charge distributions. INDO and ZINDO parameterizations are available for relatively many elements in the periodic table, but their predictions can deviate considerably from experiment. 

Originally, there have been two basic strategies for parametrization. Approximate MO methods aim at reproducing ab initio MO calculations with the same minimal basis set (MBS), whereas semiempirical MO methods attempt to reproduce experimental data. Nowadays the limitations of MBS ab initio calculations are well known and the predominant feeling is that approximate MO methods would not be useful enough in practice even if they would exactly mimic MBS ab initio calculations. Hence with the exception of PRDDO [18], current parametrizations usually adhere to the semiempirical philosophy and employ experimental reference data (or possibly, accurate high-level theoretical predictions as substitutes for experimental data see Section III.E). 

Numerous ab initio calculations with at least split-valency quality basis sets have been performed. The results are extremely sensitive to the choice of the basis set, especially the inclusion of d orbitals, and to the procedure applied, such as nth-order Moller-Plesset theory (MPn, up to n = 4) [3 to 7], coupled electron pair approximation (CEPA) [1,2, 8, 9], configuration interaction (Cl) [8, 10, 11], or SCF MO calculations [12 to 19]. For semiempirical calculations, see [20, 21 ] (EHMO), [22] (PRDDO, partial retention of diatomic differential overlap), and [23] (INDO, intermediate neglect of differential overlap). 

There have been two approaehes for parametrization of semiempirieal methods. One approach aims at reproducing ab initio MO calculations with the same minimal basis set. This approach is taken in the method of partial retention of diatomic differential overlap (PRDDO). " " The second approach aims at reproducing experimental data and/or high-level ab initio or density functional theory (DFT) calculations. Severe limitations of low-level ab initio ealeulations are well known now, especially for TM species. " As a result, parametrizations of modern semiempirical SCF MO methods follow the seeond approaeh. 

Divide and Conquer for Semiempirical MO Methods Force Fields A Brief Introduction Force Fields A General Discussion Hyperconjugation MNDO Molecular Mechanics Conjugated Systems Monte Carlo Quantum Methods for Electronic Structure PM3 PRDDO SINDOl Parameterization and Application. 

The method of partial retention of diatomic differential overlap (PRDDO) was first developed by Halgren and Lipscomb for the elements hydrogen to fluorine, and extended by Marynick and Lipscomb through the first transition series. PRDDO shares characteristics of both ab initio and semi-empirical methods. As in an ab initio approach, PRDDO calculates many two-electron integrals accurately. Like most semiempirical methods, PRDDO employs a minimal basis set (MBS) of Slater orbitals, and uses parameters to increase the accuracy of the method. 

Unlike semiempirical methods, the PRDDO/M parametrization is extremely mild. Unparametrized PRDDO/M calculations yield reasonable results, and in fact the current implementation of PRDDO/M contains no parameters for transition metals. 

MNDO MNDO/d PM3 PRDDO Semiempirical Methods Integrals and Scaling Semiempirical Methods Transition Metals. 

Currently, semiempirical methods for TM compounds are at the development stage, because the lack of accurate experimental data particularly for energies makes it mandatory that accurate ab initio calculations are used for the derivation of the necessaiy parameters. The recent progress in ab initio and DFT studies of TM compounds has given an impetus for further work in the field of semiempirical methods (see MNDO/d PRDDO Semiempirical Methods Transition Metals and SINDOl Parameterization and Application). A similar situation exists for empirical methods, where accurate parameters for MM calculations can only be derived for a well-defined subclass of TM compounds. This is one reason why the combination of quantum mechanical and MM methods has been proposed as an appropriate method for calculating large TM compounds. ... 

Other semiempirical methods which are available for theoretical studies of TM compounds are PRDDO, ZINDO (Zemer s intermediate neglect of diatomic overlap), SINDOl (symmetrically orthogonalized intermediate neglect of diatomic overlap parametrization one), and EHT (see PRDDO Semiempirical Methods Transition Metals and SINDOl Parameterization and Application). 

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