NDOS

NDO calculations use the Harircc-Kock (HK) approxirn ation to solve the Sell iddinger equation. HK melhodsdcal with several kinds of electron -electron in Leraction s. Hy iin dcrstaTidin g th esc interactions, you can appreciate differences between the NDO rn etti ods and gain in sigh t in to why the NDO approxirn ation works well or fails. 

The (juality of the vibrational fretjuen cies varies widely with the setTi i-eiTi pirical method that is used. Generally,. AMI, and l M3 are in eloser agi eem eii t with experiment than in elh ods based on C.NDO or INDO,... 

Allhougli CNOO iN IcNS icculaLc than other NDO iricLhods, there arc two common reasons for using it ... 

A projected CNDO/INDO guess uses th e com puted coefficien ts from a minimum basis set CXDCi/[NDO calculation and then... 

The complete neglect of differential overlap (CNDO) method is the simplest of the neglect of differential overlap (NDO) methods. This method models valence orbitals only using a minimal basis set of Slater type orbitals. The CNDO method has proven useful for some hydrocarbon results but little else. CNDO is still sometimes used to generate the initial guess for ah initio calculations on hydrocarbons. 

NBO (natural bond order) the name of a set of population analysis techniques NDO (neglect of differential overlap) the fundamental assumption behind many semiempirical methods... 

The second step determines the LCAO coefficients by standard methods for matrix diagonalization. In an Extended Hiickel calculation, this results in molecular orbital coefficients and orbital energies. Ab initio and NDO calculations repeat these two steps iteratively because, in addition to the integrals over atomic orbitals, the elements of the energy matrix depend upon the coefficients of the occupied orbitals. HyperChem ends the iterations when the coefficients or the computed energy no longer change the solution is then self-consistent. The method is known as Self-Consistent Field (SCF) calculation. 

The neglect of electron-electron interactions in the Extended Hiickel model has several consequences. For example, the atomic orbital binding energies are fixed and do not depend on charge density. With the more accurate NDO semi-empirical treatments, these energies are appropriately sensitive to the surrounding molecular environment. 

The Extended Hiickel method neglects all electron-electron interactions. More accurate calculations are possible with HyperChem by using methods that neglect some, but not all, of the electron-electron interactions. These methods are called Neglect of Differential Overlap or NDO methods. In some parts of the calculation they neglect the effects of any overlap density between atomic orbitals. This reduces the number of electron-electron interaction integrals to calculate, which would otherwise be too time-consuming for all but the smallest molecules. 

The NDO methods in HyperChem are built on three different approxim ation s to th e treatm en t of electron -electron in teraction s. The NDO methods are discussed in the following sections. 

The INDO method (Intermediate NDO) corrects some of the worst problems with CNDO. For example, INDO exchange integrals between electrons on the same atom need not be equal, but can depend on the orbitals involved. Though this introduces more parameters, additional computation time is negligible. INDO and MINDO/3 (Modified INDO, version 3) methods are different implementations of the same approximation. 

The five semi-empirical methods in HyperChem differ in many technical details. Treatment of electron-electron interactions is one major distinguishing feature. Another important distinguishing feature is the approach used to parameterize the methods. Based on the methods used for obtaining parameters, the NDO methods fall into two classes ... 

The choice of the NDO method depends on several factors including your previous experience and preferences. If you want to compare the results to other studies, you must use the same semi-empirical method. Since some methods can converge much more quickly than others, you might want to use a fast method to obtain an approximation of the final answer, and then a more accurate method for the final result. 

Many problems with MNDO involve cases where the NDO approximation electron-electron repulsion is most important. AMI is an improvement over MNDO, even though it uses the same basic approximation. It is generally the most accurate semi-empirical method in HyperChem and is the method of choice for most problems. Altering part of the theoretical framework (the function describing repulsion between atomic cores) and assigning new parameters improves the performance of AMI. It deals with hydrogen bonds properly, produces accurate predictions of activation barriers for many reactions, and predicts heats of formation of molecules with an error that is about 40 percent smaller than with MNDO. 

Documentation on the reliability of the different NDO methods for various applications is scattered throughout the chemistry literature. Original papers describing the methods present relevant material. The CNDO and INDO methods are discussed in books by Pople and Beveridge and by Murrell and Harget. Compilations exist for MINDO/3 and MNDO in a book by Clark. For MNDO,... 

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