MChem Master of Chemistry Computational Chemistry 2 — Questions and Answers
Question 1: In the ONIOM method, how is a large molecular system treated?
- Entirely at high-level quantum mechanics
- Divided into layers computed at different levels of theory (Correct answer)
- Using only molecular mechanics force fields
- With periodic boundary conditions throughout
Correct answer: Divided into layers computed at different levels of theory
ONIOM partitions a molecule into layers, treating the reactive core at a high QM level and the surrounding environment at a lower MM or QM level.
Question 2: What does the potential energy surface (PES) represent in a chemical reaction?
- The electrostatic potential around a molecule
- Energy as a function of nuclear coordinates (Correct answer)
- The electron density distribution
- The probability of finding an electron at a given point
Correct answer: Energy as a function of nuclear coordinates
The PES maps the total energy of a molecular system as a function of its nuclear coordinates, revealing minima, transition states, and reaction pathways.
Question 3: Which algorithm is most commonly used to locate transition states on a potential energy surface?
- Steepest descent minimization
- Nudged elastic band (NEB) or quasi-Newton eigenvector following (Correct answer)
- Monte Carlo random walk
- Simulated annealing
Correct answer: Nudged elastic band (NEB) or quasi-Newton eigenvector following
Methods like nudged elastic band and quasi-Newton eigenvector following locate saddle points (transition states) on the PES.
Question 4: In normal mode analysis, a transition state is characterized by how many imaginary vibrational frequencies?
- Zero
- Exactly one (Correct answer)
- Two
- Three or more
Correct answer: Exactly one
A true transition state has exactly one imaginary vibrational frequency corresponding to motion along the reaction coordinate.
Question 5: What is the purpose of Ewald summation in periodic boundary condition simulations?
- To integrate equations of motion more accurately
- To efficiently compute long-range electrostatic interactions (Correct answer)
- To sample configurational space faster
- To correct for quantum tunneling effects
Correct answer: To efficiently compute long-range electrostatic interactions
Ewald summation splits the long-range Coulomb sum into rapidly converging real-space and reciprocal-space components for periodic systems.
Question 6: Which semiempirical method is parameterized to reproduce heats of formation and is widely used before higher-level calculations?
- Hartree-Fock
- AM1 or PM7 (Correct answer)
- CCSD(T)
- MP2
Correct answer: AM1 or PM7
Semiempirical methods like AM1 and PM7 use empirical parameters fit to experimental data, allowing fast pre-screening before expensive ab initio calculations.
MChem Master of Chemistry Computational Chemistry 2
In the ONIOM method, how is a large molecular system treated?