HF/STO

Raghavachari has studied this problem in some detail. The following table summarizes his work as well as our own PM3 and HF/STO-3G optimizations ... 

It appears that the open isomer is the ground state structure, given the agreement between the two semi-empirical methods and HF/3-21G. HF/STO-3G predicted the structures of the two isomers reasonably well, but it does a very poor job on the energy difference. 

With these results, we see an example of a phenomenon that occurs from time to time a less accurate model chemistry will produce a better answer than more accurate ones. In our case, fortuitous cancellation of errors at the HF/STO-3G levels leads to the correct identification of the planar conformation as a transition state. However, a more accurate model chemistry is needed to properly study this system. ... 

The experimental value for the H-F bond energy is 141.2 kcal-mol. The Hartree-Fock value is in error by over 40 kcal-mol" (we ve also included the HF/ STO-3G values to indicate just how bad very low level calculations can be). However, both the MP4 and QCISD(T) values are in excellent agreement with experiment. 

Consider then a HF/STO-3G calculation on aspirin (acetylsalicylic acid) ... 

In chapter 2 I gave you the. pdb file which contains the Cartesian coordinates. We just add a couple of records to define the level of theory (HF/STO-3G), the charge (0) and spin multiplicity (1). 

X = CO2R or CN). Theoretical calculation at B3LYP/6-31G //HF/STO-3G level showed that the Si-H bond dissociation energies of H-Si(l 11) and (MesSifsSi-H are very similar, which further justifies the use of the well-established radical-based reactivity of (MesSifsSiH as a model for surface reactions. 

HF/STO-3G Amber - for R2met only). In all models the QM part consists of the two Fe centers and the first shell ligands of four formates, two imidazoles, and a few oxo, hydroxo, and/or aquo groups (see Figure 2-4). 

We also performed optimization for R2met using the three-layer ONIOM3 (B3LYP HF/STO-3G Amber). In addition to the atoms shown in Figure 2-4, an additional 45 side-chain and backbone atoms were treated at the Hartree-Fock/STO-3G level. The resultant RMS and maximum deviations are 0.23 and 0.36 A, respectively, compared to 0.34 and 0.52 A for QM MM. This indicates that the electronic effects of the protein residues, evaluated only classically in the QM MM (B3LYP Amber) treatment, can be further improved with the use of the ONIOM3 QM QM MM method. 

It has been shown that the electrostatic potentials of formamide calculated at near-Hartree-Fock (HF/6-31G ) and post-Hartree-Fock (MP2/6-31G ) levels are qualitatively similar (Politzer and Murray 1991). Both computational approaches predict the oxygen to be the preferred site for electrophilic attack (Seminario, Murray, and Politzer 1991). It is further noteworthy that SCF results obtained with minimal basis sets (e.g., HF/STO-3G and HF/STO-5G) are also in good agreement with those calculated at the higher computational levels. 

The corresponding estimate for the second eigenvalue (2s orbital energy) is —0.1789. These results are in good agreement with the actual HF/STO-3G ( Hartree-Fock method with a variational basis set of three-term Gaussians for each Slater-type orbital 10) eigenvalues eis = —2.3692 and e2s = —0.1801. 

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