Ah initio Quantum Chemical Methods

Ah initio quantum chemical calculations are primarily used to describe the electronic character of individual molecules in the gas phase. Quantum chemical methods can vary widely in their accuracy, depending on the specific approximations taken. Those most applicable to the study of ionic liquids are medium level methods such as DFT and MP2 [20]. Hartree-Fock(HF) level calculations may be carried out as a starting point or to obtain geometries but should be followed by calculations that include some level of electronic correlation. Higher level methods such as Coupled Cluster methods (ie CCSD(T)) are only just accessible, and will not be routine. They do however allow for an estimation of effects hard to recover with the lower level DFT and MP2 methods, such as dispersion, more dynamic correlation, and an estimation of other neglected effects (such as the stabilization afforded by mixing in excited electronic states). The method employed (HF DFT MP2 CCSD(T)) and sophistication of the basis set used are typically used to indicate the quality of an ab initio quantum chemical calculation. 

Quantum chemical methods can be used to determine physical properties relating to the electronic wavefunction or density, these indude vibrational firequencies 

The energy associated with internal molecular rearrangements, such as torsional rotation in alkyl chains can also be obtained and used to gain chemical insight as well as in the parameterization of force fields. 

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Using Ah Initio Quantum Chemical Methods to Study Ionic Liquids... 

Abstract A comprehensive analysis of the benefits and pitfaUs of quantum chemical methods used to determine the structures, properties, and functions of DNA and RNA fragments is presented. Main emphasis is given to the application of different ah initio quantum chemical methods. An overview of computations reveals that quantum chemical methods provide an important means to investigate structures and interactions in nucleic acids. However, judicious selection of computational approach is necessary, depending upon the nature of the problem under investigation. 

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