Rachel notes - Hybrid methods

What are the basic QM advantages disadvantages over MM

1. Predicts e- structure, reaction mechanisms, e- delocalisation

2. Computationally expensive and time consuming

What are the basic MM advantages disadvantages over QM

1. Fast and efficient

2. Lack chemical accuracy, no bond breaking predictions or e- insights

Rules for boundary/cutting through bonds

1. Furthest from the chemically active site as possible

2. Cut through single, non polar bonds. Nothing bonded to electronegative bonds. No double, triple or conj. bonds.

3. At least 3 atoms away from QM atoms. Avoids atoms involved in making/breaking bonds.

4. Avoid cutting MM charge groups. Creates artificial net charge in proximity of QM density.

5. Total charge of MM atoms replacing QM part should be 0.

What is the subtractive scheme energy equation

E Qm/MM = QM(I+L) + MM(S) + MM(I+L)

Advantages of subtractive scheme

1. Ensures no double counting

2. Easily implemented. Standard MM and QM processes can be used without modifications.

3. Availability of MM parameters for atoms in QM region.

Additive scheme energy equation

E = QM(I+L) + MM(O) + QM-MM(interaction: I, O)

Advantages of additive schemes

1. MM calculation only for O

2. QM-MM includes VdW, bonded terms and electrostatic interactions between QM and MM sites.

3. No MM parameters for QM needed as already calculated by QM.

When should you use mechanical embedding

1. Non-polar solvent used (low polarity of system - no E.S interactions).

2. When QM-MM coulomb interactions are according to MM rules.

3. Captures steric affects

Energy equation for electrostatic embedding (subtractive)

E = QM(I + L) (point charges)) + MM(S) - MM(I+L) → electrostatic term not evaluated in MM region - uses QM level theory

Energy equation for electrostatic embedding (additive)

E = QM((I + L) (point charges)) + MM (O) + QM-MM(I, O) → qm-mm terms only evaluate interactions, no electrostatics.

When should electrostatic embedding be used

1. Inner region electronic structure (QM) adapts to charge distribution changes.

2. But is more computationally demanding.

3. Boundary region of MM/QM where MM charges can QM proximity can become over polarised.