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. The correlation energy is defined as the difference between the exact electronic energy and the Hartree-Fock energy, and is attributed to reduced electronic repulsion in the exact electronic wavefunction. Which of the following statements is correct?

The correlation energy arises because Hartree-Fock theory neglects electron repulsion. correct incorrect

Correlation effects are so large that the correlation energy is often similar in magnitude to the Hartree-Fock energy. correct incorrect

In the Hartree-Fock wavefunction, electron-electron interactions are minimized as much as possible, through orbital optimization. Due to antisymmetrization of the wavefunction, close approach of electrons of same spin is prohibited in the Hartree-Fock wavefunction. correct incorrect

In some case, the correlation energy can be positive, as due to the molecular orbital approximation, the Hartree-Fock energy is highly incorrect and can end up being more negative than the exact energy. correct incorrect

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. Consider a Hartree-Fock wavefunction for water, with five doubly occupied orbitals. Three excited determinants are considered: (a) one electron is moved from the HOMO to the LUMO; (b) two electrons are moved from the HOMO to the LUMO; (c) two electrons are moved from the HOMO to the LUMO, and an additional electron is moved from the HOMO-1 to a high-lying vacant orbital. Which of the following statements concerning their contributions to the energy in a configuration interaction calculation is true?

Determinant (b) will introduce the largest stabilization, as it is a double excitation. correct incorrect

Because only one electron is moved to a less stable orbital, determinant (a) will play the largest role. correct incorrect

Without doing a calculation, it is impossible to tell which determinant will have the largest role to play. correct incorrect

The Hartree-Fock wavefunction is not very accurate, so the more changes, the better. Determinant (c), which is most different from the Hartree-Fock determinant, will play the largest role. correct incorrect

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. Calculations are carried out on the neon atom and a dimer consisting of two neon atoms separated by 60 Å, at two different levels of theory (a) and (b). The energies obtained are (a) -128.7961 and -257.5924 hartree, and (b) -128.7919 and -257.5688 hartree. What can you conclude from these calculations?

Total energies of this type tell us little beyond the energy required to remove the electrons from the neon nuclei. correct incorrect

Method (a) is better because it returns lower energies in both cases. correct incorrect

Method (b) returns an energy for the system with two neon atoms that is less negative than twice the energy for two neon atoms; this must be right since we know neon cannot bond to neon. correct incorrect

Method (a) is size-consistent (the energy for two distant neon atoms is exactly twice the energy for one neon atom), while method (b) is not size-consistent - twice -128.7919 hartree is -257.5838 hartree. correct incorrect

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. Which of the following reactions are isodesmic?

1. H₂O → H + OH;
2. CH₃Cl + propane → i-propyl chloride + methane;
3. benzene + 3 H₂ → cyclohexane;
4. HOOH + CH₃OOF → HOOF + CH₃OOH.

Are any of these reactions not isogyric?

All of them are isodesmic, but reaction (i) is not isogyric. correct incorrect

Reactions (i) and (iii) are isodesmic, all the reactions are isogyric. correct incorrect

Reactions (ii) and (iv) are isodesmic; all the reactions except (i) are isogyric. correct incorrect

Reaction (ii), (iii) and (iv) are isodesmic; reaction (i) is neither isodesmic nor isogyric. correct incorrect

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. The CCSD(T) method (coupled-cluster theory with single and double excitations, and perturbative treatment of triples) is one of the most accurate quantum chemical methods. Which of the following is not a drawback of this method?

Size-inconsistency. correct incorrect

Huge computational expense, especially for larger molecules. correct incorrect

High requirements on the size of the basis set. correct incorrect

A tendency to yield incorrect energies when significant static correlation is present, as for stretched bonds. correct incorrect

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. In density functional theory, one part of the overall energy is difficult to calculate based on the electron density function alone, leading to the use of 'orbitals' similar to those used Hartree-Fock theory. Which is this problematic part of the energy?

The electron-electron repulsion energy. correct incorrect

The kinetic energy of the electrons. correct incorrect

The interaction energy of the electrons with the nuclei. correct incorrect

The exchange and correlation energy. correct incorrect

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. Which of the following is the exception?

AM1 correct incorrect

BP86 correct incorrect

MP2 correct incorrect

B3LYP correct incorrect

6-31G correct incorrect

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. Synthetic chemists wish to prepare a new molecule, and wish to assess its stability. Computation is first used to try to predict its energy relative to a starting product or a possible decomposition product. Calculations with the following methods (and basis sets) are used. Which do you expect to be most reliable?

B3LYP/6-31G correct incorrect

CCSD(T)/cc-pVTZ correct incorrect

MP2/cc-pVDZ correct incorrect

AM1 correct incorrect

HF/STO-3G correct incorrect

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. The Hartree-Fock energy for the ammonia molecule with the cc-pVTZ basis set and for a given structure is -56.1749 hartree. Which of the following could be the CCSD(T) energy at the same structure, using the same basis set?

-56.4242 hartree. correct incorrect

-59.9531 hartree. correct incorrect

-56.1393 hartree. correct incorrect

-56.1802 hartree. correct incorrect

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. A DFT calculation is carried out on diamond using a periodically repeating model. The repeating unit cell contains eight atoms. Compared to a calculation using a standard quantum chemical code on the molecule adamantane (C₁₀H₁₆), using the same DFT functional and a similar basis set, would you expect the calculation to take:

Much longer– after all, we are modelling a whole crystal here. correct incorrect

Much less long, because DFT is more efficient for crystals. correct incorrect

Roughly the same amount of time. correct incorrect

Almost exactly the same amount of time. correct incorrect