Pople correction

The Pople corrections for higher excitations in CISD calculations result in a method called quadratic (Q) CISD, which can be regarded as an approximation to the coupled-cluster methods discussed in a separate section below. QCISD is available in the Gaussian series of programs, along with analytical derivatives and a method for the estimation of triple excitations called QCISD(T). Note that in Gaussian, CISD on open-shell molecules is performed with UHF MOs. Therefore, for such molecules, the methods just named should really be called UQCISD or UQCISD(T), respectively. 

By systematically applying a series of corrections to approximate solutions of the Schroedinger equation the Pople group has anived at a family of computational protocols that include an early method Gl, more recent methods, G2 and G3, and their variants by which one can anive at themiochemical energies and enthalpies of formation, Af and that rival exper imental accuracy. The important thing... 

To obtain the G2 value of Eq we add five corrections to the starting energy, [MP4/6-31 lG(d,p)] and then add the zero point energy to obtain the ground-state energy from the energy at the bottom of the potential well. In Pople s notation these additive terms are... 

OPW (orthogonalized plane wave) a band-structure computation method P89 (Perdew 1986) a gradient corrected DFT method parallel computer a computer with more than one CPU Pariser-Parr-Pople (PPP) a simple semiempirical method PCM (polarized continuum method) method for including solvation effects in ah initio calculations... 

A closer look at the original data published by Johnson, Gill, and Pople, 1993, reveals that use of the gradient-corrected LYP correlation functional instead of VWN increases atomization energies for non-hydride species quite significantly, while those containing hydrogen atoms are reduced. This rather systematic trend is portrayed in Table 9-2 for a few example cases. 

Johnson, B. G., Gonzales, C. A., Gill, P. M. W., Pople, J. A., 1994, A Density Functional Study of the Simplest Hydrogen Abstraction Reaction. Effect of Self-Interaction Correction , Chem. Phys. Lett., 221, 100. 

In summary, VH F demonstrates the same pattern of solvent dependence as does 2/h h. However, all the subtleties seem to be enhanced. Usually 2/H F decreases in solvents of higher dielectric strength, but an appropriate dipole orientation with respect to the H—C—F group can lead to the opposite result as is observed in vinyl fluoride. This situation is perhaps most likely to occur in mono-fluoro compounds where the fluorine is the principal contributor to the molecular dipole. In either case the electric field effect as postulated with the Pople expression for the contact term produces the correct prediction. 

A modification of G2 by Pople and co-workers was deemed sufficiently comprehensive tliat it is known simply as G3, and its steps are also outlined in Table 7.6. G3 is more accurate titan G2, witli an error for the 148-molecule heat-of-formation test set of 0.9 kcal mol . It is also more efficient, typically being about twice as fast. A particular improvement of G3 over G2 is associated with improved basis sets for tlie third-row nontransition elements (Curtiss et al. 2001). As with G2, a number of minor to major variations of G3 have been proposed to either improve its efficiency or increase its accuracy over a smaller subset of chemical space, e.g., the G3-RAD method of Henry, Sullivan, and Radom (2003) for particular application to radical thermochemistry, the G3(MP2) model of Curtiss et al. (1999), which reduces computational cost by computing basis-set-extension corrections at the MP2 level instead of the MP4 level, and the G3B3 model of Baboul et al. (1999), which employs B3LYP structures and frequencies. 

Considering the proliferation of MO calculations in recent years, it is remarkable how few calculations have been reported for these heterocycles. Frontier orbitals of several betaines have been calculated using the Hiickel method and electroselectivity correctly predicted on the basis of the orbital symmetry.Similar results have been obtained for pyridinium-3-olates (427) using the Pariser- Parr Pople (PPP) method. The CNDO... 

Roth and coworkers41 42 have chosen carefully recalibrated force fields to predict reliable heats of formation with errors intended by the authors to be as small as 0.5 kcal mol"1. They started with the MM2 force field of Allinger98 and added to this parameters for the C(sp2)—C(sp2) and the C(cyclopropyl)—C(sp2) single bonds of reference compounds such as substituted 1,3-butadienes and vinylcyclopropanes41. Particular care was given to the correct description of the torsion potential of the reference compounds. The modified MM2 force field (MM2ERW) developed by Roth and coworkers describes polyenes and cyclopolyenes in terms of localized bond structures without any reference to quantum chemical methods such as the Pariser-Parr-Pople (PPP) approach (see Section II.C)41. 

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