Semi-empirical molecular statistical

The MEP at the molecular surface has been used for many QSAR and QSPR applications. Quantum mechanically calculated MEPs are more detailed and accurate at the important areas of the surface than those derived from net atomic charges and are therefore usually preferable [Ij. However, any of the techniques based on MEPs calculated from net atomic charges can be used for full quantum mechanical calculations, and vice versa. The best-known descriptors based on the statistics of the MEP at the molecular surface are those introduced by Murray and Politzer [44]. These were originally formulated for DFT calculations using an isodensity surface. They have also been used very extensively with semi-empirical MO techniques and solvent-accessible surfaces [1, 2]. The charged polar surface area (CPSA) descriptors proposed by Stanton and Jurs [45] are also based on charges derived from semi-empirical MO calculations. 

Hpp describes the primary system by a quantum-chemical method. The choice is dictated by the system size and the purpose of the calculation. Two approaches of using a finite computer budget are found If an expensive ab-initio or density functional method is used the number of configurations that can be afforded is limited. Hence, the computationally intensive Hamiltonians are mostly used in geometry optimization (molecular mechanics) problems (see, e. g., [66]). The second approach is to use cheaper and less accurate semi-empirical methods. This is the only choice when many conformations are to be evaluated, i. e., when molecular dynamics or Monte Carlo calculations with meaningful statistical sampling are to be performed. The drawback of semi-empirical methods is that they may be inaccurate to the extent that they produce qualitatively incorrect results, so that their applicability to a given problem has to be established first [67]. 

This relationship might be available in the form of experimental data, or it could be represented by a model. Models are usually based on experimental data, but they also possess predictive capabilities. That is, they are expected not only to reproduce the correlated data, but also to generate data over reasonable ranges of conditions. Although many PVT models are semi-empirical, some are based on theoretical principles such as molecular thermodynamics and statistical thermodynamics. No single PVT correlation exists that can accurately predict all properties for diverse substances over wide ranges of temperature, pressure, density, and composition. Nevertheless, a number of models have demonstrated their usefulness for many applications. 

Rather, flie assignment is more serious wifli intermolecular interaction potential used. For simple molecules, empirical model potential such as fliose based on Lennard-Jones potential and even hard-sphere potential can be used. But, for complex molecules, potential function and related parameter value should be determined by some theoretical calculations. For example, contribution of hydrogen-bond interaction is highly large to the total interaction for such molecules as HjO, alcohols etc., one can produce semi-empirical potential based on quantum-chemical molecular orbital calculation. Molecular ensemble design is now complex unified mefliod, which contains both quantum chemical and statistical mechanical calculations. 

In principle, there are two strategies for molecular design. The first strategy is to start from the first principle, i.e., from quantum mechanics and statistical mechanics to predict the property of unknown materials. Up to now, however, it is still impossible to use this strategy to solve the most of complicated problems in materials exploration work. The second way is a semi-empirical one. It starts from the known data of some molecules and some molecular descriptors (including quantum chemical parameters) to find semi-empirical rules, and then use these empirical rules to predict the property of unknown molecules. The second way is already practicable for molecular design or new materials exploration. 

---