Ab-Initio Molecular Orbital Methods

Question 1

Ab-Initio Method

Answer 1

Methods (including HF-SCF) with no empirical parameters
(Ab-Initio = from the beginning)

Question 2

What do Gaussian-type orbitals (GTO’s) have?

Answer 2

Gaussian decay (e^(-{r^2))
One GTO is not as accurate as one STO, but the ERI’s over GTO’s have analytical formulae and are thus faster to compute than ERI’s over STO’s

Question 3

Contracted GTO

Answer 3

Fixed linear combination of GTO’s to mimic an STO for better accuracy

Question 4

Zeta basis sets

Answer 4

Single zeta: 1 contracted GTO per AO
Double zeta: 2 contracted GTO per valence AO
Triple zeta: 3 contracted GTO per valence AO

Question 5

Polarization functions

Answer 5

Basis functions having one ‘angular momentum’ value higher than the valence orbitals

eg. d functions for B, C, N, O, F

Question 6

What do polarization functions do?

Answer 6

Help build optimal MO’s for more accuracy

Question 7

Diffuse functions

Answer 7

Basis functions that are very diffuse (small zeta values)

Question 8

What do diffuse (++ or avg) functions do?

Answer 8

Help describe weakly bound electrons, especially lone pairs and anions

Question 9

General performance of HF-SCF (energetics)

Answer 9

• good for internal rotations (4 kJ/mol error)
• bad for bond rearrangements (50 kJ/mol error)
• terrible for reactions that make radicals

Question 10

General performance of HF-SCF (geometries)

Answer 10

• rij too short by 0.03 A (C-C) and 0.015 A (C-H)
• theta(ij) to +/- 2 degrees

Question 11

General performance of HF-SCF (partial charges)

Answer 11

• dipole moments 10-25% too large