Electronic structure calculations geometric optimization

The basic output of an electronic structure calculation is the electronic and the total energy of the system for a certain nuclear geometry. When the energy is minimized as a function of a number of internuclear distances or bond angles, this is referred to as a (constrained) geometry optimization. This will normally give the equilibrium structure under the restraints of the geometrical model. 

For all but the few smallest clusters, the number of possible structures is virtually unlimited. In order to be able to treat the larger systems, quite restrictive assumptions about their geometry has to be made. For those clusters where well-defined equilibrium structures do exist, these are likely to possess a non-trivial point-group symmetry (in many cases the highest possible symmetry). It therefore seemed justified to focus the study on high-symmetric systems. Symmetry can also be used to simplify the calculation of electronic structure, and reduces the number of geometrical degrees of freedom to be optimized. 

The calculation of the electronic structure of PANI and adsorption complexes (AC) PANI- oxygen has been carried out with complete optimization of the geometric bond parameters. 

The paper [8] includes results of investigating electron mechanisms of the impact of active particles, radicals, hydrated electrons artificially generated by plasma on the behavior of cyanide complexes of zinc in water solutions. The above investigation was conducted using quantum chemistry methods. Quantum-chemical calculation of electron structure of the complexes Zn(CN)42 4EP-20H- with complete optimization of all geometric parameters [9] was performed. 

At the conclusion of a geometric optimization calculation, we have the equilibrium positions of all the atomic nuclei, as well as the overall electron density distributed in space (x, y, z). Many important properties, especially for an isolated single molecule at absolute zero temperature, can be obtained by solving the quantum mechanical or the molecular mechanical equations. Only the former method can produce electronic properties, such as electron distributions and dipole moments, but both methods can produce structural and energy properties. 

Sato and coworkers [25-27] investigated the electronic and geometric structure of the coronene monoanion, CaaHn (Fig. 2). ESR observation in solution exhibited no JT effect down to 183 K. JT distorted structure was obtained using HF/6-31G calculation. The optimized structure of the monoanionic state has C2/, symmetry ( Bg electronic state) for the energy minimum (JT stabilization energy of 297 meV) and D2h for the transition structures (energy barrier of ca 0.2 meV between C2/, minima). 

Let us first examine the ground state (X W) of the vinoxy radical, where a variety of familiar HF, MP, and DFT electronic structure methods are applicable. Specifically, we employ the B3LYP/6-311-H-G level of theory for direct comparison with the parent closed-shell acetaldehyde molecule 2 discussed in Section 7.1. Table 7.3 displays optimized geometrical parameters calculated at this level (column 3), showing the satisfactory agreement with previous theoretical and experimental studies. 

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