Core repulsion function modification

Despite these modifications there remain a number of well-documented problems with the AM1/PM3 core-repulsion function [37] which has resulted in further refinements. For example, Jorgensen and co-workers have developed the PDDG (pair-wise distance directed Gaussian) PM3 and MNDO methods which display improved accuracy over standard NDDO parameterisations [38], However, for methods which include d-orbitals (e.g. MNDO/d [23,24], AMl/d [25] and AMI [39,40]) it has been found that to obtain the correct balance between attractive and repulsive Coulomb interactions requires an additional adjustable parameter p (previously evaluated using the one-centre two-electron integral Gss, Eq. 5-7), which is used in the evaluation of the two-centre two-electron integrals (Eq. 5-8). 

A second modification recently described by Repasky, Chandrasekhar, and Jorgensen (2002) focuses on improving the core-repulsion functions in MNDO and PM3. In particular, they define a pairwise distance directed Gaussian function (PDDG) to compute a contribution to the nuclear repulsion energy between atoms A and B as... 

AMI is currently one of the most commonly used of the Dewar-type methods. It was the next semiempirical method introduced by Dewar and coworkers in 1985 following MNDO. It is simply an extension, a modification to and also a reparameterization of the MNDO method. AMI differs from MNDO by mainly two ways. The first difference is the modification of the core repulsion function. The second one is the parameterization of the overlap terms (3s and (3p, and Slater-type orbital exponents (s and (p on the same atom independently, instead of setting them equal as in MNDO. MNDO had a very strong tendency to overestimate repulsions between atoms when they are at approximately their van der Waals distance apart. To overcome this hydrogen bond problem, the net electrostatic repulsion term of MNDO, J RAH) given by equation (8.2), was modified in MNDO/H to be... 

The PDDG/PM3 and PDDGAINDO methods are modifications of PM3 and MNDO that add a certain function, called the pairwise distance directed Gaussian (PDDG) function, containing additional parameters, to the core-repulsion function, thereby significantly increasing the accuracy of these methods [M. P. Repasky et al., J. Comput. Chem., 23, 1601 (2002) I. Tubert-Brohman et al., J. Comput. Chem. 25,138 (2004) J. Chem. Theory Comput., 1, 817 (2005)]. 

Later, two modifications of MNDO were introduced [102,235]. The most prominent of these are the Austin model 1 (AMI) by Dewar et al. [246] and the parametric method 3 (PM3) by Stewart [248]. In principle MNDO, AMI and PM3 methods differ only in the parametrization and in the empirical function /mat in the core-core repulsion... 

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