Block correlated second order perturbation theory with a generalized valence bond reference function.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate electronic structure calculations are crucial for understanding molecular properties and reactivity.
• Existing methods like MP2 and CASSCF have limitations for systems with strong multi-reference character or large active spaces.

Purpose of the Study:

• To introduce and validate a novel computational method, GVB-BCPT2, for electronic structure calculations.
• To assess the performance of GVB-BCPT2 compared to established methods like MP2 and CASSCF.

Main Methods:

• Development of a block correlated second-order perturbation theory using a generalized valence bond (GVB) reference function.
• Defining blocks as geminals or individual spin orbitals and constructing a zeroth-order Hamiltonian.
• Direct, iterative calculation of GVB-BCPT2 energy, ensuring size consistency.

Main Results:

• GVB-BCPT2 demonstrated superior performance compared to MP2 for systems exhibiting significant multi-reference character.
• The method provided reasonably accurate results for systems with large active spaces, surpassing the capabilities of CASSCF-based methods.
• Applied successfully to calculate equilibrium distances, spectroscopic constants, conformational energies, and bond-breaking profiles for various molecules.

Conclusions:

• GVB-BCPT2 is a promising computational tool for studying complex chemical systems where traditional methods fall short.
• The method offers a balance of accuracy and efficiency, particularly for systems with strong electron correlation.
• This approach extends the applicability of perturbation theory to a wider range of challenging quantum chemical problems.