Møller–Plesset Perturbation Theory (MP2)

 Møller–Plesset Perturbation Theory (MP2)

• Hartree–Fock (HF) → ignores electron correlation  
• CI → includes correlation but very expensive

So we need a method that is

 More accurate than HF

Cheaper than CI

That method = MP2

HF gives you a rough answer
MP2 gives you a correction

Idea of Perturbation Theory

We start from HF Hamiltonian:

$$$$

H₀ = Hartree–Fock part                        V  = perturbation (correlation) 

MP2 (Second-order Perturbation

Key Concept

E_HF​ = Hartree–Fock energy                            E² = correlation correction

Example