Semi-empirical Calculations on Molecules

5 Semi-empirical Calculations on Molecules. - The number of semi-empirical packages that can be employed routinely to estimate the a, and y tensors is proliferating, but they can still be classified into a few broad groups  

PM3 or MNDO hamiltonians for calculations on organic molecules. The MOPAC and AMPAC packages are the usual sourees for these programs. The exeited state spectrum calculated in the NDDO based methods is usually inferior to that obtained in CNDO(S) or ZINDO/INDO, but sealing is sometimes used to overcome this diffieulty. It is essential to use SDCI or better if any reasonable treatment of the y-hyperpolarizabiUty is to be achieved. 

In the following review of publieations we attempt to identify the type of calculation that has been employed in eaeh case. 

Grummt et al have applied this method to the study of the y-hyperpolariz-abdity of hydroxystdbazofium salts and their betaines. 

Wang et al have used the AMl/FF approach for static properties and ZlNDO/SCl to study the -hyperpolarizabdity of some azulene derivatives. High values are obtained and it is concluded that the azulene ring is a more efficient conjugation bridge between donor and acceptor than either benzene or thiophene. They use the same methods in a study of push-pull polyenes containing nonaromatic cycfic olefines and for tetra-substituted benzenes and pyrazines. 

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