MP2 levels

Based on the equation found in Problem 23, estimate the total energy of n-pentanoic acid by extrapolation to 5 carbon atoms. Carry out the calculation at the 6-3IG MP2 level in the GAMESS implementation and determine the 9c difference between the G.AMKSS calculation and the extrapolated estimate. 

Allylic and benzylic radicals are also stabilized by both acceptor and donor substituents. As shown in Table 12.5, theoretical calculations at the MP2 level indicate that substituents at the 2-position are only slightly less elfective than 1-substituents in the... 

In a higher level energy calculation, values for the energy computed using the more accurate procedure appear shortly after the Hartree-Fock energy. Here is the output from a formaldehyde calculation done at the MP2 level (RMP2 replaces RHF in the route section ... 

The number following EUMP2 is the predicted energy at the MP2 level, approximately -114.16666 hartrees. 

All of the model chemistries predict acetaldehyde to the lower energy isomer. The methods including electron correlation all produce good estimates of the isomerization energy. However, it turns out that the MP2 value is fortuitously good increasing the basis set size would produce a poorer result at the MP2 level. For the... 

Hartree-Fock theory does quite a poor job of predicting the structures and frequencies for these compounds. It produces highly distorted structures in all three cases, and its computed frequencies bear little resemblance to the experimental observations. MP2 theory generally does better for the structures, although it fails to located a distorted structure for Na F3. The frequencies computed at the MP2 level also vary widely from experiment. 

Gaussian-2 theory adds some additional corrections to the G1 final result. The major term is a correction at the MP2 level, described in the next step ... 

Subsequent calculations at the MP2 level locate the two transition structures like those suggested. In this case, Intrinsic Reaction Coordinate (IRC) calculations were used to confirm that these transition structures do in fact connect the minima in question we ll look at this kind of calculation in detail later in this chapter. 

All of the available levels of theory appear in the table of results. For example, a scan run at the MP2 level will also include the Flartree-Fock energies at each point. 

In order to save computation time, set up the second and subsequent jobs to extract the electron density from the checkpoint file by using the Geom=Checlcpoint and Densiiy=(Checkpoint/AP2) keywords in the route section. You will also need to include Den iiy=MP2 for the first job, which specifies that the population analysis should be performed using the electron density computed at the MP2 level (the default is to use the Hartree-Fock density). 

We ran an SCRF single point energy calculation for gauche dichloroethane conformers in cyclohexane (e=2.0), using the Onsager model at the Hartree-Fock and MP2 levels of theory (flfl=3.65) and using the IPCM model at the B3LYP level. The 6-31+G(d) basis set was used for all jobs. We also ran gas phase calculations for both conformations at the same model chemistries, and an IPCM calculation for the trans conformation (SCRF=Dipole calculations are not necessary for the trans conformation since it has no dipole moment). 

As the plot of AE indicates, the energy difference between the two forms decreases in more polar solvents, and becomes nearly zero in acetonitrile. The left plot illustrates the fact that the IPCM model (at the B3LYP/6-31+G(d) level of theory) does a much better job of reproducing the observed solvent effect than the two Onsager SCRF models. In contrast, the Onsager model at the MP2 level treats the solvated systems more accurately than it does the gas phase system, leading to a poorer value for the solvent effect. ... 

Table 11.2 H2O geometry as a function of basis set at the MP2 level of theory...

The variation at the CCSD(T) level is shown in Table 11.3, with the ehange relative to the MP2 level given as A values. Additional eorrelation with the CCSD(T) method gives only small changes relative to the MP2 level, and the effeet of higher-order eorrelation diminishes as the basis set is enlarged. For H2O the CCSD(T) method is virtually indistingable from CCSDT. ... 

It should also be noted that the effect of electron correlation at the MP2 level (relative to HF) is largely independent of the basis set, but there is a significant coupling between... 

These effects are shown in Tables 11.27-11.30. The energetic changes due to optimization at the MP2 level, relative to the HF geometries, are less than 0.7 kcal/mol (Table 11.27). 

Table 11.29 Energies relative to 1 calculated at the MP2 level with different basis sets, using MP2/6-31G(d,p) optimized geometries...

By assuming additivity in the style of the G2 procedure (Section 5.5), the CCSD(T)/ 6-31G(d,p) results may be combined with the changes due to basis set enlargement to 6-31 lG(2df,2pd) at the MP2 level and the zero-point energy corrections calculated at the MP2/6-31G(d,p) level. The results are shown in Table 11.31. From the observed accuracy of 2 kcal/mol for structures 2-8, the energetics of the species 9-11 may be assumed to be reliable to the same level of accuracy. 

Tables 12.1-12.3 below give some examples of the magnitude of each term for two bimolecular reactions (Diels-Alder and Sn2 reactions, forming either one or two molecules as the product) and a unimolecular rearrangement (Claisen reaction). All values have been calculated at the MP2 level with the 6-31G(d) basis for the Diels-Alder and Claisen reactions, and the 6-31+G(d) basis for the S l reaction. The values are given in kcal/mol at a temperature of 300 K (RT = 0.60 kcal/mol).

Ab initio calculations at SCF and MP2 levels with the 6-31G+G(d,p) basis [97MI(11)153] predieted a elear preferenee for the oxo speeies of 2-azahypoxanthine (55) and the predominanee of the 7/7 form 55a both in the gas phase and in aqueous solution. 

The vibrational spectra of 1,2-dithiole-3-thione 46 and 1,2-dithiol-3-one 47 were computed at the DFT and MP2 levels (Scheme 31) [98VS77]. Most remarkably, the uniformly scaled MP2 fundamentals are in better agreement with experimental data than the corresponding DFT frequencies. 

Since HF calculations have a tendency to underestimate the N—N and the C—N bond lengths in triazoles [98JPC(A)620, 98JPC(A) 10348], the structural parameters should be computed at least at the DFT or MP2 levels. This is particularly true if electron-donating substituents are attached to the ring. Nitrogen NMR shielding tensors were computed for a set of methylated triazoles and tetrazoles but will be discussed in the context of tetrazoles (cf. Section IV,B). 

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