Quantum chemical procedures

Barone, V., Arnaud, 1997, Diels-Alder Reactions An Assessment of Quantum Chemical Procedures , J. Chem. Phys., 106, 8727. 

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. 

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 a series of papers various semiempirical quantum chemical procedures were examined for their usefulness to describe ion-ligand interactions (for a review see 7)). 

Barone, B. Amaud, R. Diels-Alder reactions an assessment of quantum chemical procedures, J. Chem. Phys. 1997,106, 8727-8732. 

The multi-reference coupled electron pair approximation (MRCEPA) has a long pedigree as one of the more rigorous quantum chemical procedures. A recent development, the state specific version (SS-MRCEPA) has been... 

An outline of the quantum chemical procedure used to determine the interaction energies needed in Eq. (24) is as follows [see Sum and Sandler (1999a,b) for details of the calculations]. 

The QUANTUM theoretical characterization of the molecular structure of polycyclic benzenoid aromatic hydrocarbons (PAHs) and the relationships of structure to the physical and chemical properties of PAHs are problems that have been of concern to theoreticians (and experimentalists) for more than 50 years. In general, quantum chemical procedures can be used successfully to correlate kinetic and thermodynamic data for PAHs. These procedures are usually restricted to the it systems of the PAHs and normally seem to yield very good results because (1) the it system properties are described accurately by quantum mechanical calculations and (2) the energetics of a given type of reaction in a group of related PAHs is mainly... 

As calculated dipole moments in comparison with experimental values represent sensitive tests of the qualities of the molecular wavefunctions obtained from a particular quantum-chemical procedure, correlation (15) is a basis for confidence in theoretically calculated quantities with the CNDO/S-method. 

NMR is a powerful tool for the determination of structures from first principles and the chemical shift is the most important NMR parameter in structural analysis. For estimating the relationship between chemical structures and chemical shifts three possibilities exist the calculation of the chemical shift values by empirical methods [137], the computation by quantum chemical procedures, e.g., with the IGLO-method (Individual Gauge for Localized Orbitals [ 129]), or the use of large compilations of NMR spectra and the associated chemical structures. The access to relevant reference data for identical or similar compounds can facilitate the assignment process enormously. Reference data may assist by reducing the amount of experimental and/or interpretive effort required or increase confidence in the suggested structure. 

It is interesting to compare the possibilities and errors of different hybrid QM/MM schemes. The careful examination and comparison of link atom and LSCF techniques was performed in Ref. [128] using the CHARMM force field [114] and the AMI method [143] as a quantum chemical procedure. In the case of the link atom procedure two options were used QQ - the link atom does not interact with the MM subsystem and HQ - link atom interacts with all MM atoms. The main conclusion of this consideration is that the LSCF and the link atom schemes are of similar quality. The error in the proton affinity determination induced by these schemes is several kcal/mol. It is noteworthy that all the schemes work rather badly in description of conformational properties of n-butane. The large charge on the MM atoms in the proximity of the QM subsystem (especially on the boundary atom) cause significant errors in the proton affinity estimates for all methods (especially, in the case of the LSCF approach where the error can be of tens of kcal/mol). This is not surprising since the stability and transferability of intrabond one- and two-electron density matrix elements Eq. (19) is broken here. It proves that the simple electrostatic model is not well appropriate for these schemes and that a detailed analysis of the... 

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