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Physical > Computational > Flashcards

Computational Flashcards

1
Q

Which model is most suitable for TM?

A

B3LYP/6-31G*

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2
Q

Which model is most suitable for unsaturated organics?

A

MP2/6-31G* or B3LYP/6-31G*

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2
Q

Which model is most suitable for weakly associated dimers?

A

HF/G-311+G*

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3
Q

Which model is most suitable for compounds with electronegative elements or lone pairs?

A

MP2/6-311+G*

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4
Q

MM

How are electrons treated?

A

They are ignored

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5
Q

Advantages of MM?

2 points

A
  1. Quick not computationally demanding
  2. Applicable to very large systems
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6
Q

Disadvantages of MM?

4 points

A
  1. Electrons are ignored
  2. Bond energies, molecular orbitals, electron densities, excited states are not calculated
  3. Primary function of force field is structure optimisation
  4. Calculations depend on the force field used.
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7
Q

Describe MD calculations

A
  1. Take forces calculated on atoms
  2. Apply Newton 2nd Law
  3. Define time step
  4. Output molecular trajectory
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8
Q

MD

What is a typical time step?

A

1 fs

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9
Q

MD

What do force fields define?

A

Potential energy surface

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10
Q

MD

How are force fields calculated?

A

From the gradient of the potential energy surface

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11
Q

What is the principle of Ab initio calculation?

A

MO theory approach to finding solutions to Schrödinger’s equation

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12
Q

Ab initio

How are electrons treated?

A

Explicitly

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13
Q

Ab initio

What does Hamiltonian operator describe?

A

KE & PE terms of matter

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14
Q

Ab inito

How is the Hamiltonian operator approximated?

A

Using a level of theory Hartree-Fock or DFT

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15
Q

Schrödinger equation

What is E?

A

Proportionality constant (the Eigenvalue)

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16
Q

Schrödinger equation

What does the wavefunction describe?

A

Size and shapes of atomic and molecular orbitals

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17
Q

Schrödinger equation

How is the wavefunction approximated?

A

Using a basis set. Linear combination of simple functions like Gaussian

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18
Q

Born-Oppenheimer approximation of Hamiltonian operator

What happens to KE of electron

A

Worked out

19
Q

Born-Oppenheimer approximation of Hamiltonian operator

What happens to KE of nucleus

A

=0, as molecule is fixed in space for geometry optimisation

20
Q

Born-Oppenheimer approximation of Hamiltonian operator

What hapens to PE of electron to electon

A

Worked out

21
Q

Born-Oppenheimer approximation of Hamiltonian operator

What happens to PE of nucleus to nucleus

A

Calculated using Coulomb’s law

22
Q

Born-Oppenheimer approximation of Hamiltonian operator

What happens to PE of nucleus to electron

A

Calculated

23
Q

Which level(s) of theory approximate electron to electron correlation

A

DFT, Hybrid DFT

24
Q

Which level(s) of theory ignore electron to electron correlation

A

HF theory

25
Q

Which level(s) of theory approximate electron to electron exchange

A

DFT

26
Q

Which level(s) of theory get exact electron to electron exchange

A

HF theory, Hydrid DFT

27
Q

Describe Hartree theory

A

Replaces the insolvable many electron problem with the solvable single electron problem. Each electron modelled as a moving in an effective uniform field

28
Q

What is the disadvantage of HF theory

A

Missing electron correlation energy

29
Q

For what molecules is the missing electron correlation energy significant?

A

Significant for molecules with high electron density (aromatics)

30
Q

How many variable s per electron are used in HF?

A

4

3 positional + spin

31
Q

Variation theorem

Define basis set

A

Collection of Gaussian functions

32
Q

Variaiton theorem

Define polarisation function

A

One higher angular momentum function

33
Q

Variation theorem

Describe the process of variation theorem

A
  1. Generate a trial wavefunction based on random weighting coefficients
  2. Determine its energy
  3. Vary weighting coefficients to get a new wavefunction. Is the new energy lower?
  4. Repeat until the electronic ground state (minimum is found)
34
Q

Basis sets

What is the meaning of dash?

A

Treat core electrons separate from valence electrons

35
Q

Basis sets

What is the meaning of star?

A

Polarisation function (two stars — polarisation on the hydrogen as well as heavy; one star — only heavy, not hydrogen)

36
Q

Basis sets

What is the meaning of plus?

A

Diffuse function. Larger versions of ‘s’ and ‘p’ orbitals. Important for atoms that carry extra charge

37
Q

What is the difference between semi-empirical calculations and ab initio?

A
  1. Remove core electrons from calculations
  2. Use minimal basis set
  3. Zero differential overlap
38
Q

What is zero differential overlap?

A

Ovelap integral between many atoms assumed to be zero

39
Q

MD

What is to be calculated to get the nature of the stationary point on the potential energy surface?

A

Nedd to calculate 3N-6 force constant

40
Q

MD, PES stationary points

What makes a minimum?

A

If all 3N-6 vibrational frequencies are real numbers

41
Q

MD, PES stationary points

What makes a saddle point?

A

If one freq is imaginary and the rest are real, 1st order transition state

42
Q

What is the main difficulty in calculating NMR with DFT?

A

Need specially constructued split-valence basis sets with enhanced descriptors for core electrons

43
Q

What can calculating EPR with DFT show?

A

By comparing plots from experimental you can verify the model and then assume that the molecular orbitals are shown correctly

44
Q

Electron correlation

A

Describes how the electric field/movement of one electron is influenced by the electric field/movement of all other electrons. It’s a manifestation of the many-body effect.

45
Q

Electron exchange

A

Describes the energy lowering obtained when spin up (alpha) electrons pair upwith spin down (beta) electrons

Physical (4 decks)

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