Organic molecules electron-correlated calculations

The disadvantage of ah initio methods is that they are expensive. These methods often take enormous amounts of computer CPU time, memory, and disk space. The HF method scales as N, where N is the number of basis functions. This means that a calculation twice as big takes 16 times as long (2" ) to complete. Correlated calculations often scale much worse than this. In practice, extremely accurate solutions are only obtainable when the molecule contains a dozen electrons or less. However, results with an accuracy rivaling that of many experimental techniques can be obtained for moderate-size organic molecules. The minimally correlated methods, such as MP2 and GVB, are often used when correlation is important to the description of large molecules. 

While calculations at the Hartree-Fock level provide results which can be used for the adequate interpretion of the IR spectra of most organic molecules, the practice today is to include electron correlation in the calculation for molecules containing up to 6 to 10 first row atoms. Inclusion of electron correlation provides some improvement in the agreement between the observed and calculated spectra. The most widely used method for including correlation is the M0ller-Plesset method limited to the second order (MP2). 

A systematic analysis of the electrostatic interactions in the crystals of 40 rigid organic molecules was undertaken by Price and coworkers (D. S. Coombes et al. 1996). In this work, distributed (i.e., local) multipoles up to hexadecapoles, obtained from SCF calculations with 6-31G basis sets, scaled by a factor of 0.9 to allow for the omission of electron correlation, are used in the evaluation of the electrostatic interactions. The experimental lattice constants and structures are reproduced successfully, the former to within a few percent of the experimental... 

Jia et al. also studied the cluster continuum method using PCM with the HF/ 6-31+G(d), HF/6-3U++G(d,p), and B3LYP/6-3U++G(d,p) levels of theory for Eq. (13). For a data set of five organic acids, they found that the accuracy of the pKa calculations increases as the number of explicit water molecules increases from 0 to 3 [93]. In this study, relative pKa values were computed, so that lack of electron correlation in the gas-phase calculation apparently cancelled. 

Except in the highest symmetry cases, ab initio calculation of zero-field splitting in organic molecules requires the use of H , an operator that has only a two-electron part. Then the heavy computation involving two-electron terms cannot be avoided regardless of what spin-orbit Hamiltonian is used. These calculations are difficult because the correlation of the electrons has to be described very well before the zero-field splitting parameters are calculated accurately. Of the... 

---