Computational quantum mechanics calculating properties

In addition to the obvious structural information, vibrational spectra can also be obtained from both semi-empirical and ab initio calculations. Computer-generated IR and Raman spectra from ab initio calculations have already proved useful in the analysis of chloroaluminate ionic liquids [19]. Other useful information derived from quantum mechanical calculations include and chemical shifts, quadru-pole coupling constants, thermochemical properties, electron densities, bond energies, ionization potentials and electron affinities. As semiempirical and ab initio methods are improved over time, it is likely that investigators will come to consider theoretical calculations to be a routine procedure. 

MolSurf parameters [33] are descriptors derived from quantum mechanical calculations. These descriptors are computed at a surface of constant electron density, with which a very fine description of the properties of a molecule at the Van der Waals surface can be obtained. They describe various electrostatic properties such as hydrogen-bonding strengths and polarizability, as well as Lewis base and acid strengths. MolSurf parameters are computed using the following protocol. 

The electronic structure and physical properties of any molecule can in principle be determined by quantum-mechanical calculations. However, only in the last 20 years, with the availability and aid of computers, has it become possible to solve the necessary equations without recourse to rough approximations and dubious simplifications2. Computational chemistry is now an established part of the chemist s armoury. It can be used as an analytical tool in the same sense that an NMR spectrometer or X-ray diffractometer can be used to rationalize the structure of a known molecule. Its true place, however, is a predictive one. Therefore, it is of special interest to predict molecular structures and physical properties and compare these values with experimentally obtained data. Moreover, quantum-mechanical computations are a very powerful tool in order to elucidate and understand intrinsic bond properties of individual species. 

Molecular dynamics simulations, with quantum-mechanically derived energy and forces, can provide valuable insights into the dynamics and structure of systems in which electronic excitations or bond breaking processes are important. In these cases, conventional techniques with classical analytical potentials, are not appropriate. Since the quantum mechanical calculation has to be performed many times, one at each time step, the choice of a computationally fast method is crucial. Moreover, the method should be able to simulate electronic excitations and breaking or forming of bonds, in order to provide a proper treatment of those properties for which classical potentials fail. 

The great diversity of the new ligands makes it difficult to identify which effect plays the main role and in which step. It is still not clear whether the rate-determining step and the selectivity-determining step coincide, or whether the selectivity is determined by the HRh(CO)(alkene)(diphosphine) intermediate, species never observed experimentally. High-level quantum mechanical calculations on the whole molecular system are needed to be able to properly describe metal-phosphorous bond properties and its effect on the energy barriers, but this is not possible with the computational resources currently available. 

Schrodinger equation. When the molecule is too large and difficult for quantum mechanical calculations, or the molecule interacts with many other molecules or an external field, we turn to the methods of molecular mechanics with empirical force fields. We compute and obtain numerical values of the partition functions, instead of precise formulas. The computation of thermodynamic properties proceeds by using a number of techniques, of which the most prominent are the molecular dynamics and the Monte Carlo methods. 

In recent years, great progress has been made in quantum-mechanical calculations of the properties of small organic molecules by so-called ab initio methods, which means calculations from basic physical theory using only fundamental constants, without calibration from known molecular constants. Calculations that are calibrated by one or more known properties and then used to compute other properties are called semiempirical calculations. 

Recent progress in computational hardware and the development of efficient algorithms have assisted the routine development of molecular quantum-mechanical calculations. New semiempirical methods calculate realistic quantum-chemical molecular quantities in a relatively short computational time frame. Quantum-chemical calculations are thus an attractive source for molecular descriptors that can express all of the electronic and geometric properties of molecules and their interactions. Quantum-chemical methods can be applied to QSARs by direct derivation of electronic descriptors from the molecular wave function. 

Before any computational study on molecular properties can be carried out, a molecular model needs to be established. It can be based on an appropriate crystal structure or derived using any other technique that can produce a valid model for a given compound, whether or not it has been prepared. Molecular mechanics is one such technique and, primarily for reasons of computational simplicity and efficiency, it is the most widely used. Quantum mechanical modeling of metal complexes with ab-initio or semi-empirical methods often remains prohibitive because these methods are so computationally intensive. The approximations that are introduced in order to reduce central processing unit (CPU) time and allow quantum mechanical calculations to be used routinely are often severe and such calculations are then less reliable. 

At the theoretical level, full quantum mechanical calculations on biologic macromolecules are not computationally feasible, nor would they be particularly helpful in understanding macro-molecular properties without proper inclusion of the solvent water or other biologic matrix on which these properties so intimately depend. However, ab initio quantum mechanical calculations on smaller systems that represent crucial steps in an enzymic reaction, for example, can be helpful in understanding specific processes within macromolecules or in estimating intermolecular forces and stereochemical effects in molecular mechanics simulations that are not experimentally accessible. 

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