Hartree-Fock calculation conclusions

This mechanistic question is one of the examples of the success of density functional theory methods in organometallic chemistry. Earlier work on the reaction mechanism could not discriminate between the two alternatives. Analysis of the different orbitals based on extended Hiickel calculations came to the result that the [3+2] pathway is more likely, but could not exclude the possibility of a [2+2] pathway [13]. Similar conclusions where obtained from the results of Hartree-Fock calculations in combination with QCISD(T) single point calculations [21], Attempts to use Ru04 as a model for osmium tetraoxide indicated that the formation of an oxetane is less favorable compared to the [3+2] pathway, but still possible [22, 23],... 

As a conclusion, Hartree-Fock calculations are seen to be qualitatively compatible with the simple models (ligand field theory for dd-transitions, and the weak coupling model for CT-transitions). However, the ab initio work strongly suggests that the results be situated in a different conceptual framework. 

Finally, Halls and Schlegel36 studied a whole series of molecules for which they calculated the infrared intensities with various (Hartree-Fock and Cl-like as well as LDA, GGA, and hybrid density-functional) methods. Their general conclusion is that the density-functional calculations, essentially independent of which type of functional was used, lead to considerably better agreement with the most accurate Cl results than do the Hartree-Fock calculations. Although one may recognize some improvement of the hybrid functionals over the LDA or GGA approximations, it is interesting to observe that this is only small. This may be taken as a consequence of the fact that the hybrid forms have been developed first of all in order to produce accurate total energies and less for the calculation of other properties. 

The truncation procedure explored in the present smdy is described in detail in section 2. An analysis of the orbital expansion coefficients for the ground state of the BF molecule is presented in section 3, where the truncated basis sets employed in the present study are defined. The results of both matrix Hartree-Fock calculations and second-order many-body perturbation theory studies are given in section 4 together with a discussion of the properties of the truncated basis sets. The final section, section 5, contains a discussion of the results and conclusions are given. 

Recently, a similar study was performed using multicenter structures of the H- He- H biradical system (2-5) to check if the conclusions obtained for dinuclear complexes can be extended to polynuclear complexes [34]. Although there is no experimental information for such hypothetical compounds, highly accurate post Hartree-Fock calculations were employed to obtain reference values for comparison. 

C-N and N-N couplings have been analyzed by coupled Hartree-Fock calculations, and the conclusions can help to explain couplings to H, P, Si, metals etc. If neither of the coupled atoms bears a lone pair with 5 character, A (NX) like X(CC) is positive and increases with 5 character in the bond orbitals (e.g. 5/ <5/7 <5/ ). The Fc term normally dominates, but the orbital and spin-dipolar terms increase with n bond order and become relatively large for triple bonds. These terms depend on and so increase with effective nuclear charge of the... 

However, until today no systematic comparison of methods based on MpUer-Plesset perturbation (MP) and Coupled Cluster theory, the SOPPA or multiconfigurational linear response theory has been presented. The present study is a first attempt to remedy this situation. Calculations of the rotational g factor of HF, H2O, NH3 and CH4 were carried out at the level of Hartree-Fock (SCF) and multiconfigurational Hartree-Fock (MCSCF) linear response theory, the SOPPA and SOPPA(CCSD) [40], MpUer-Plesset perturbation theory to second (MP2), third (MP3) and fourth order without the triples contributions (MP4SDQ) and finally coupled cluster singles and doubles theory. The same basis sets and geometries were employed in all calculations for a given molecule. The results obtained with the different methods are therefore for the first time direct comparable and consistent conclusions about the performance of the different methods can be made. 

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