MChem Master of Chemistry Computational Chemistry 1 — Questions and Answers
Question 1: Which method uses the Born-Oppenheimer approximation to separate nuclear and electronic motion?
- Molecular mechanics
- Ab initio quantum chemistry (Correct answer)
- Monte Carlo simulation
- Molecular dynamics
Correct answer: Ab initio quantum chemistry
Ab initio quantum chemistry relies on the Born-Oppenheimer approximation to treat nuclear and electronic degrees of freedom separately.
Question 2: In density functional theory (DFT), the exchange-correlation functional approximates which quantity?
- Kinetic energy of electrons
- Nuclear repulsion energy
- Many-body exchange and correlation effects (Correct answer)
- Electrostatic Coulomb energy
Correct answer: Many-body exchange and correlation effects
The exchange-correlation functional in DFT approximates the complex many-body exchange and correlation effects that cannot be computed exactly.
Question 3: What is the primary advantage of the B3LYP hybrid functional in DFT calculations?
- It uses no empirical parameters
- It combines exact Hartree-Fock exchange with DFT correlation for improved accuracy (Correct answer)
- It eliminates basis set superposition error
- It is exact for multi-reference systems
Correct answer: It combines exact Hartree-Fock exchange with DFT correlation for improved accuracy
B3LYP mixes a portion of exact HF exchange with DFT exchange and correlation functionals, yielding improved thermochemical accuracy.
Question 4: In molecular dynamics (MD) simulations, what does the NPT ensemble maintain constant?
- Number of particles, volume, and temperature
- Number of particles, pressure, and temperature (Correct answer)
- Energy, volume, and temperature
- Number of particles, energy, and pressure
Correct answer: Number of particles, pressure, and temperature
The NPT (isothermal-isobaric) ensemble keeps the number of particles (N), pressure (P), and temperature (T) constant.
Question 5: Which basis set type uses Slater-type orbitals approximated by Gaussian functions?
- Plane-wave basis sets
- Pople-style split-valence basis sets (e.g., 6-31G) (Correct answer)
- Numerical atomic orbital basis sets
- Natural bond orbital basis sets
Correct answer: Pople-style split-valence basis sets (e.g., 6-31G)
Pople-style basis sets like 6-31G approximate Slater-type atomic orbitals using contracted Gaussian functions for computational efficiency.
Question 6: What does the term 'basis set superposition error' (BSSE) describe in computational chemistry?
- Error from truncating the Hamiltonian operator
- Artificial lowering of energy when fragments share basis functions (Correct answer)
- Inaccuracy in computing nuclear repulsion
- Error from using an incomplete exchange-correlation functional
Correct answer: Artificial lowering of energy when fragments share basis functions
BSSE arises when interacting fragments use each other's basis functions, artificially lowering the computed interaction energy.
MChem Master of Chemistry Computational Chemistry 1
Which method uses the Born-Oppenheimer approximation to separate nuclear and electronic motion?