Hybrid correlation models based on active-space partitioning: seeking accurate O(N5) ab initio methods for bond...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Moller-Plesset second-order (MP2) perturbation theory is a cost-effective ab initio method for electron correlation.
• Standard MP2 methods exhibit divergent behavior in potential energy curves at large interatomic separations when using restricted Hartree-Fock orbitals.
• Previous work introduced a hybrid MP2-CCSD method to improve MP2 potential energy curves while maintaining MP2's computational scaling.

Purpose of the Study:

• To expand hybrid methodology to other schemes beyond MP2-CCSD.
• To investigate a new, improved MP2-CCSD method and other O(N5) Epstein-Nesbet pair correlation theories.
• To evaluate the accuracy of these methods for molecular properties and compare them with full configuration interaction results.

Main Methods:

• Development and application of a new hybrid MP2-CCSD method.
• Investigation of other O(N5) methods based on Epstein-Nesbet pair correlation theory.
• Calculation of nonparallelity errors and spectroscopic constants for molecules (BH, HF, H2O, CH+, CH4, Li2) using the 6-31G* basis set.

Main Results:

• The new hybrid MP2-CCSD method demonstrates superior accuracy compared to standard MP2.
• The improved method significantly outperforms MP2 at both large interatomic separations and near equilibrium geometries.
• The investigated O(N5) methods, including the new hybrid MP2-CCSD, were compared against full configuration interaction results.

Conclusions:

• The developed hybrid MP2-CCSD method offers a significant improvement over standard MP2 for calculating potential energy curves.
• This enhanced ab initio approach provides accurate results across various interatomic distances, addressing MP2's limitations.
• Among the considered O(N5) methods, the new hybrid MP2-CCSD method stands out for its accuracy and performance.