Hartree-Fock ab initio methods

Historically, Hartree-Fock methods were the first to attack many-particle problems, with considerable success for atoms and molecules. Cluster calculations can be employed to study impurities in this scheme. Ab initio Hartree-Fock methods are very computationally intensive, however, and thus restricted to small clusters. Correlation effects are neglected. The use of expanded basis sets (only a first step towards configuration-interaction analysis) rapidly increases computation time. 

Even ab initio Hartree-Fock methods can become very expensive for large systems. In these cases, the semiempirical methods are the ones generally applied. In these methods, some of the integrals are neglected and others are replaced using empirical data. 

L-B. With polarizable charges obtained by the ab initio Hartree-Fock method. 

Hobza P, Kabelac M, Sponer J, Mejzlik P, Vondrasek J (1997) Performance of empirical potentials (AMBER, CFF95, CVFF, CHARMM, OPS, POLTEV), semiemprical quantum chemical methods (AMI, MNDO/M, PM3) and ab initio Hartree-Fock method for interaction of DNA bases comparison of nonempirical beyond Hartree-Fock results, J Comp Chem, 18 1136-1150... 

Table 4.6. Atomic charges calculated by least squares fit to electric potentials calculated by ab-initio Hartree-Fock methods [329] ...

From the above results it follows that the most stable geometry is an alternating one at HF level with nonzero E. Its actual value, however, is too large in comparison with the ptical absorption maximum observed below 2 eV ( ). This problem of the theoretical overestimation of the energy gap is a known drawback of the ab initio Hartree-Fock method (11) and serves as one of the... 

The IR intensity of a normal mode is, from Eq. (79), proportional to (3, /3Q)2. One approach would be to determine experimentally a set of bond moment (electro-optical) parameters from which the (3. /dQ) could be calculated (Person and Zerbi, 1982). For molecules with as low a symmetry as peptides this presents serious difficulties. Our approach has been to use Eq. (76), dii/dQ = 2. (dfi/dSi)Lia, to calculate the dfi/dSi from NMA by ab initio Hartree-Fock methods, and to use the Lia obtained from our empirical force field for the particular polypeptide. 

The investigation of water interaction with the cluster of hydroxyfuller-ene Qo [ Jio was carried out in the frameworks of ab initio Hartree-Fock method in the basis 6-3IG. To simplify the calculation model the cluster with the rotation axis of fifth order of firllerene molecule was applied. 

Thus, comparison of calculation results with experimental data demonstrates ab initio Hartree-Fock method in the 6-3 IG basis set provides qualitatively correct conclusion on hydrogen bond energies for systems considered. This model was applied to investigate the interaction between water and hydroxyfullerene cluster Qo. Hydroxyfullerene is a proper object for quantum-chemical modeling of interaction between activated carbon nanoparticle and water. At the same time hydroxyfullerene is of great interest because of its possible applications in medicine, for water disinfection, and for polishing nanosurfaces. 

Calculations were performed with the ab initio Hartree-Fock method (4) and in some cases with methods incorporating electron correlation such as MP2 (5) or BLYP (6), Geometry optimizations were initally done with relativistic effective core potential polarized valence double-zeta bases (7) and then refined with standard 6-3IG bases (8). NMR shieldings were evaluated using the GIAO-SCF method (9) and 6-3IG bases. We used the quantum chemical software GAMESS (10) and GAUSSIAN (11). 

Quantum computational methods are mainly used in systems for which electronic properties are of interest, such as molecular orbitals. Nuclear Magnetic Resonance (NMR) spectra, and polarizability. Usually an optimization process is carried out [3]. It consists in finding the structure which exhibits the lowest energy. The prerequisite in manipulating approximations to solve the Schrodinger equation gives rise to different approaches. They can be roughly classified into three major types ab initio (Hartree-Fock method and derivatives), density functional theory (DFT), and semiempirical methods, ab initio is a Latin locution which means from the... 

It has to be mentioned that the simple procedure used in CCM MSINDO of an averaging of two-center integrals between the central atom of the interaction region and atoms at the boundaries of its WSC is not possible for ab-initio Hartree-Fock methods due to the presence of three- and four-center integrals. 

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