Gaussian theory accuracy

In fact, we have already used a modeling strategy when Po(AU) was approximated as a Gaussian. This led to the second-order perturbation theory, which is only of limited accuracy. A simple extension of this approach is to represent Pq(AU) as a linear combination of n Gaussian functions, p, (AU), with different mean values and variances [40]... 

The field of quantum chemistry has seen tremendous development over the last thirty years. Thanks to high-accuracy models such as coupled-cluster theory and standardized, widely available program packages such as Gaussian 98, what was once merely an esoteric tool of a few specialists has evolved into an indispensable source of knowledge for both the prediction and the interpretation of chemical phenomena. With the development of reduced scaling algorithms for coupled cluster... 

There has been much recent progress in the application of density functional theory (DFT) to the calculation of shift tensors, and several methods are presently available. The sum-over-state (SOS) DFT method developed by Malkin et al. (70) does not explicitly include the current density, but it has been parametrized to improve numerical accuracy. Ziegler and coworkers have described a GIAO-DFT method (71) that is available as part of the Amsterdam density functional package (72). An alternate method developed by Cheeseman and co-workers (73) is implemented in Gaussian 94 (74). 

Gaussian-2 (G2) theory Ideally, a sucessful method for computation of thermochemical data has several features (1) it should be applicable to any molecular system in an unambiguous manner, (2) it must be computationally efficient so that it can be widely applied, (3) it should be able to reproduce known experimental data to a prescribed accuracy, and (4) it should give similar accuracy for species for which the data are not available or for which experimental uncertainties are large. The Gaussian-n methods were developed with these objectives in mind. Gaussian-1 (Gl) theory was the first in this series.27 28 We will not cover Gl theory in this chapter because it was replaced by G2 theory, which eliminated several deficiencies in Gl, and because G2 is currently the most widely used method of this series. 

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