RPA natural orbitals and their application to post-Hartree-Fock electronic structure methods

Area of Science:

• Computational chemistry
• Quantum chemistry
• Electronic structure theory

Background:

• Accurate calculation of electron correlation is crucial for predicting molecular properties.
• Post-Hartree-Fock methods are computationally expensive, limiting their application.
• Approximate natural orbitals can potentially reduce computational cost.

Purpose of the Study:

• To develop and validate a method for approximating post-Hartree-Fock correlation energies.
• To assess the efficiency of using Random Phase Approximation (RPA) natural orbitals.
• To provide a computationally feasible approach for correlated wave function methods.

Main Methods:

• Approximation of post-Hartree-Fock correlation energies using RPA natural orbitals.
• Application to benchmark systems: helium atom, H2, F2, and diamond.
• Comparison with full configuration interaction (FCI) calculations and literature values.

Main Results:

• RPA natural orbitals rapidly converge the MP2 correlation energy for benchmark systems.
• Calculated FCI energies for He and H2 show excellent agreement with experimental and literature data.
• The proposed method demonstrates a good balance between accuracy and computational cost.

Conclusions:

• The RPA natural orbital method provides a viable compromise for approximating correlation energies.
• This method is expected to be particularly useful for surface chemistry studies.
• It offers an efficient basis for correlated wave function based methods at moderate computational expense.