Gaussian theoretical models

Gaussian includes many different model chemistries. The theoretical model chemistries in Gaussian have been subjected to the testing procedure described previously and so may be recommended for general use with any system for which they are computationally feasible. 

Model chemistries are characterized by the combination of theoretical procedure and basis set. Every calculation performed with Gaussian must specify the desired theoretical model chemistry in addition to specifying the molecular system to consider and which results to compute for it. 

Mebel, A. M., Morokuma, K., lin, M. C., 1995, Modification of the GAUSSIAN-2 Theoretical Model The Use of Coupled-Cluster Energies, Density Functional Geometries, and Frequencies , J. Chem. Phys., 103, 7414. 

A basic theoretical model for flexible polymers is the Gaussian chain which assumes N ideal beads with intramolecular distance between them following a Gaussian distribution, so that the mean quadratic distance between two beads separated by n-1 ideal and not correlated bonds is given by [ 15,20]... 

At this point let us return to the aluminum content data of Table 5.1. The skewed shape that is evident in all of Figs 5.2—5.5 makes a Gaussian distribution inappropriate as a theoretical model for (raw) aluminum content of such PET samples. But as is often the case with right skewed data, considering the logarithms of the original measurement creates a scale where a normal distribution is more plausible as a representation of the phenomenon under... 

Since the retention of the template on the imprinted polymers strongly depends on the sample load (see later Fig. 5.25), the theoretical models describing the various dispersion processes are not applicable. Nevertheless, on an imprinted CSP for L-PA, the least retained enantiomer, D-PA, elutes as a fairly Gaussian peak (Fig. 5.3), which should be governed by the same band broadening mechanisms that are considered in the models. A measure of the dispersion of a Gaussian peak is the deviation from its mean value, which is reflected in the reduced plate height as ... 

There is, at present, no realistic theoretical model of the effect of geometrical disorder. A proper model must take into account the complicated orientation dependence of the matrix element (K) [63g], as well as the distribution of positions and orientations of the CTM. The simple treatment adopted by the Gaussian Disorder Model predicts a dependence of the prefactor mobility (//q) on the geometrical disorder parameter Z, namely //q oc exp(i2 ) [63a], and this relationship has been used often to analyze experimental data. The comparison of TTA and TAPC at low concentrations disagrees with this prediction, and it appears to be untenable a priori anyway [60b, 63e]. A worthy challenge for future work will be to construct a model with enough realism to have predictive value. Recent reports indicate progress in this direction [641,o]. 

The inversion double well potential in ammonia is a difficult problem that has been the object of numerous theoretical studies over the years [2-12], We chose to model this potential as the superposition of a harmonic part [modeling its overall shape to the zeroth Hamiltonian Ho<2Uq2), and two Gaussian terms (modeling the barrier), supplemented by three small corrections to the overall shape, of fourth, sixth, and eighth powers in (q2) ... 

---