Quantum Chemistry

Question 1

Define a minimal basis set

Answer 1

A set of basis functions containing the minimum number of functions needed to describe the electronic structure of a molecule with reasonable accuracy

Question 2

How many basis sets does C2H5OH have in STO-3G?

Answer 2

21
C - 10
H - 6
O - 5

Question 2

What is a split valence basis set?
Give 2 types of split valence basis sets

Answer 2

Using different types of basis functions to describe core and valence electrons to lower the cost of calculation
You’re able to do this because core electrons likely have a lesser impact on bonding so don’t require as detailed of a calculation
e.g 6-31G , 3-21G

Question 3

What does it mean to augment a basis set and give 2 examples?

Answer 3

• Adding additional basis functions to an existing basis set to improve its accuracy in describing the electronic structure
  e.g 6-31G adding an additional set of diffuse functions
  3.g 6-31 + G inclusion of additional polarisation functions to improve description of electron correlation

Question 4

What is density functional theory?

Answer 4

Solving Shrödinger for many electron systems by using Hohenberg-Kohn theory which allows total energy of a system to be expressed as a function go electron density E=E[ρ(r)]
inaccuracy: strong correlation effects

Question 5

Compare DFT with Wave function methods

Answer 5

Most ab initio WF methods are based on Hartree Fock (approximating using a Slater determinant composed of occupied spin orbitals, spin and spacial functions ie LCAO and using mean field theory)
DFT: solves shrödinger based on electron density using hohenberg-Kohn and is better as it accounts for electron correlation

Question 6

What is meant by HF is an example of the Variational principle?

Answer 6

The approximate energy calculated by HF must be higher than the true ground state of the system
More basis sets gives more accurate properties but will never reach the true value