Quantum chemical energy procedures

The qualitative molecular state model [1-4], the application of which is strongly recommended, can usually be transparently parametrized by physical measurement data such as vertical ionization energies [1, 2, 5, 6], Therefore, the potentials at the individual Si nuclear centers no longer have to be circumscribed by ill-defined quantities such as electronegativity or hard and soft , but are advantageously characterized by their effective nuclear charges introduced by Slater [7], nowadays inherent in many quantum-chemical calculation procedures. 

Fig. 1.1 (a) In traditional quantum chemical methods the potential energy surface (PES) is characterized in a pointwise fashion. Starting from an initial geometry, optimization routines are applied to localize the nearest stationary point (minimum or transition state). Which point of the PES results from this procedure mainly depends on the choice of the initial configuration. The system can get trapped easily in local minima without ever arriving at the global minimum struc-... 

For chemical purposes, substitution of total energy hypersurfaces by those based on the heat of formation seems more reasonable, with the difference given by the zero point energy corrections. However, their calculations from first principles can be rather cumbersome (12) and, moreover, for a given variation of some nuclear coordinates it usually can be assumed that the change in zero point energy is small compared to that of the total energy. On the other hand, se eral semiempirical quantum chemical procedures which are appropriately parametrized often yield satisfactory approximations for molecular heats of formation (10) and, therefore, AH hypersurfaces have become rather common. 

In this section, we discuss theoretical methods, which can be applied for calculations of photoabsorption and PL spectra of silica and germania nanoparticles. We start with the choice of model cluster simulating these materials and point defects in them and consider methods for geometry optimization in the ground and excited electronic states (Subsection 2.1). This is followed by the description of more advanced quantum chemical methods for accurate calculations of excitation energies (Subsection 2.2) and the section is completed by the discussion on the theoretical procedure used for predicting vibronic spectra associated with point defects (Subsection 2.3). 

The above equation is very conveniently used as the computation of the total energy is the standard quantum-chemical procedure. However, a purely theoretical problem arises when using monomer-centered basis set for evaluation of EA and EB according to (20.1) The intermolecular interaction energy will suffer from what is known as basis set superposition error (BSSE) [3], In order to overcome this unphysical effect which usually manifests itself in too negative interaction energies, one frequently applies the so-called counterpoise correction [4],... 

In molecular-mechanics calculations, the atoms are considered to move in a force field defined by an energy function based on classical (rather than quantum) mechanics. Thus, the energy of a given molecular conformation is not calculated in an iterative SCF procedure, as in quantum-chemical approaches, but rather uses an analytical formula based on effective potentials. 

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