Coupled Cluster Theory

 

Coupled Cluster Theory (CC Theory)

Introduction to CC Theory

Coupled Cluster (CC) is one of the most accurate methods for calculating electronic structure.

It systematically includes electron correlation and is widely used in quantum chemistry.

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

Electron Correlation & Configuration Interaction (CI)

 

Electron Correlation & Configuration Interaction (CI)

Do electrons in a molecule really move independently?

Answer: No
They repel each other and try to avoid each other all the time.

What Hartree–Fock (HF) Assumes

In Hartree–Fock (SCF):

• Each electron moves in an average field of others
• Instant repulsion ignored
• Only one Slater determinant is used

Slater Determinant

 

Slater Determinant

The Problem We Need to Solve

Electrons are:

·         Indistinguishable

·         Follow Pauli Exclusion Principle

·         Wavefunction must be antisymmetric