QCISD (quadratic CISD) 113, 117, 119 QSAR (quantitative structure-activity relationships) 695-706, 710, 711 cross-validation 701 deriving equation 698-70 discriminant analysis 703-5 interpreting equation 702 neural networks 703-5 principal components regression 706 -property relationship 695, 702 selecting compounds for analysis 697-8 QSPR (quantitative structure-property relationship) 695, 702 quadratic region 283-4 quadrupole 76, 181, 183, 185-6, 196 quantitative structure-activity see QSAR quantum mechanics future role 160-1 [Pg.756]

Methods based on Density Functional Theory also include some electron correlation effects (we ll consider them a bit later in this chapter). Of the traditional post-SCF methods, we ll be primarily using MP2, MP4, QCISD and QCISDfO in this work. [Pg.114]

We can compute all of the results except those in the first row by running just three jobs QCISD(T,E4T] calculations on HF and fluorine and a Hartree-Fock calculation on hydrogen (with only one electron, the electron correlation energy is zero). Note that the E4T option to the QCISDfT) keyword requests that the triples computation be included in the component MP4 calculation as well as in the QCISD calculation (they are not needed or computed by default). [Pg.115]

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. [Pg.116]

Including triply excited configurations is often needed in order to obtain very accurate results with MP4, QCISD or CCSD (see Appendix A for some of the computational details). The following example illustrates this effect. [Pg.118]

The structure of ozone is a well-known pathological case for electronic structure theory. Prior to the QCI and coupled cluster methods, it proved very difficult to model accurately. The following table summarized the results of geometry optimizations of ozone, performed at the MP2, QCISD and QCISD(T) levels using the 6-31G(d) basis set [Pg.118]

Compute the isomerization energy between acetaldehyde and ethylene oxide at STP with the QCISD(T)/6-31G(d) model chemistry, and compare the performance of the various model chemistries. Use HF/6-31G(d) to compute the thermal energy corrections. Remember to specify the scaling factor via the Freq=Recxllso option. (Note that we have already optimized the stmcture of acetaldehyde.) [Pg.129]

MP4 and the two QCISD methods, the predicted isomerization energy v/ould continue to converge toward the experimental value as the basis set size increases. [Pg.130]

The B3LYP and QCISD values are in good agreement with one another and with thi experimental observations. Both favor the resonance form with the radical centerec mainly on the C2 carbon. Therefore, we will use the B3LYP/6-31G(d) mode chemistry for the remainder of this study. [Pg.131]

Compute the isotropic hyperfine coupling constant for each of the atoms in HNCN with the HF, MP2, MP4(SDQ) and QCISD methods, using the D95(d,p) basis set Make sure that the population analysis for each job uses the proper electron density by including the Density=Current keyword in the route section. Also, include the 5D keyword in each job s route sectionfas was done in the original study). [Pg.136]

There is rather poor agreement between the QCISD values and all of the lower levels of theory this is a case where the successive MP orders converge rather slowly. Note that the QCISD values differ only a bit from Carmichael s QCISD(TQ) results. It turns out also that MP4(SDTQ) does not improve on the MP4(SDQ) values (accordingly, we chose the cheaper method for this exercise). [Pg.137]

The short summary of these results is that none of these model chemistries is very accurate at modeling this process in toto. Some of them achieve good results on either the component dissociation energies or the final value of AH, but no method does well for all of them. Not even QCISD(T) at the very large 6-311+G(3df) basis set Ls adequate. A compound energy method is required to successfully address thus problem. We will see such a solution in the next chapter. [Pg.138]

See also in sourсe #XX -- [ Pg.312 ]

See also in sourсe #XX -- [ Pg.26 ]

See also in sourсe #XX -- [ Pg.207 , Pg.208 ]

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