Geometry Optimizations

1
Geometry Optimizations

• Gaussian Workshop Day 3

2
Optimization Sequence

• 1) Calculate 2nd Derivative Force Constant
  Matrix Hessian (Requires Energy)
• 2) Generate Step
• 3) If step acceptable calculate Energy and
  Gradients at new coordinates
• 4) Check for convergence
• 5) If not restart at 2 using updated Hessian.

3
Convergence Criteria

• Max Force less than Critical Value
• RMS Force less than Critical Value
• Max Step less than Critical Value
• And RMS Step less than Critical Value
• Or Max/RMS Force less than 1/100 critical value
  even if step is large

4
Berny Optimization Step 0
5
Optimization Step 1
6
Optimization Step 2 etc.
7
What Can I Do now?

• Calculate reaction DG.
• Calculate difference between Singlet and Triplet
  Oxygen.
• Compare two stable conformations of a molecule
  (cis/trans, staggered/eclipsed etc.)
• Gas Phase Ionization Energies.

8
Homework

• Reading Should be up to Chapter 4 by next week.
• Homework Calculate DGf for Water
• H2 ½ O2 -gt H2O
• Remember 1 Hartree 4.359e-18 J
• Begin some of your own problems, bring in
  difficulties.
• NMR?