Water correlation energy

In order to gather more information about this problem, it was deemed worthwhile to follow the energetics of the alkylation reaction of water by methyl-, ethyl-, and fluo-roethyldiazonium ions. The main goal of these calculations was to establish whether transition-state calculations can provide information about hard versus soft electrophilic character of these species.12 Computations at Hartree-Fock and MP2 level were performed using the 6-31G basis set. It was found that both at the Hartree-Fock level and when correlation energy affects were included, the ethyl and fluoroethyl species do not show the presence of a transition state, while the methyl species show a small transition state. It was concluded that transition state computations cannot shed light on the characters of these species. 

In the nitrogen molecule, we found no significant difference in the CT energies between using NOs and canonical CAS orbitals over the potential curve, unlike in the H2O case. However, it is still important to optimize the orbitals, as the CASCI-based L-CTD does not yield a correct potential curve. Finally, as in the water calculations, the L-CTSD(2) approximation recovers most of L-CTSD correlation energy across the potential energy curve. 

Yamaki, D., Koch, H., Ten-no, S. Basis set limits of the second order Moller-Plesset correlation energies of water, methane, acetylene, ethylene, and benzene. J. Chem. Phys. 2007, 127, 144104. 

Components of the correlation energy of the ground state of the water molecule using Gaussian basis sets of different quality ... 

Table 5 Contribution of triple-excitations and quadruple-excitations to the correlation energy of the water molecule at its equilibrium nuclear geometry...

Baum, J. O., and Finney, J. L., An SCF-Cl study of the water dimer potential surface atui the effects of including the correlation energy, the basis set superposition error, and the Davidson correction, Mol. Phys. 55, 1097-1108 (1985). 

The methods have been very successful, but they do suffer drawbacks. The lack of parameters for many elements seriously limits the types of problems to which the methods can be applied and their accuracy for certain problems is not very good (for example, both MNDO and AM 1 do not well describe water-water interactions). There are also questions about the theoretical foundations of the models. The parameterization is performed using experimental data at a temperature of 298K and implicitly includes vibrational and correlation information about the state of the system. Therefore, the parameterization is used, in part, to compensate for quantities that the HF method cannot, by itself, account for. But what happens if vibrational or correlation energy calculations are performed With these caveats and if one can be certain of their accuracy in given circumstances, the methods are very useful as calculations can be performed with them much more quickly than ab initio QM calculations. Even so, they are probably still too computationally intensive to treat complete condensed phase systems in a routine manner. 

S. J. Chakravorty and E. R. Davidson, The water dimer correlation energy calculations,... 

The results of some preliminary calculations of the second order correlation energy component for the water molecule, a prototype non-linear polyatomic system, are displayed in Table 5. AE2 is again the difference between the correlation energy estimate obtained with a given basis set and that obtained with the 0 sp H s set. It can be seen that, whereas the 0 sp H sp OH sp HH sp set gave a lower energy than the 0 spd H.sp set in matrix Hartree-Fock calculations, the situation is reversed for the correlation energy studies. 

Some second order correlation energy components for the water... 

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