Variational coupled cluster for ground and excited states.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Coupled-cluster (CC) methods are essential for accurate electronic structure calculations, involving solving non-linear equations for amplitudes.
• While typically used for ground states, CC methods can access higher-energy solutions, potentially representing excited states.
• Paired double excitations (pCCD) offer a simplified yet relevant model for studying CC method behavior.

Purpose of the Study:

• To explore the energy landscape of variational coupled-cluster (VCC) methods.
• To compare the performance of variational pCCD (VpCCD) against traditional pCCD.
• To evaluate these methods against doubly occupied configuration interaction (CI) using various reference orbitals.

Main Methods:

• Investigated two model systems: linear and rectangular H4 molecules, subjected to weak and strong correlation regimes.
• Employed variational pCCD (VpCCD) and traditional pCCD.
• Used reference Slater determinants from ground-state, excited-state Hartree-Fock, and state-specific optimized orbitals.

Main Results:

• The study analyzes the energy landscape structure of VpCCD and compares it with traditional pCCD.
• Performance was gauged against doubly occupied CI, revealing insights into the accuracy of VpCCD with different orbital choices.
• The influence of spatial symmetry breaking on the VpCCD and pCCD methods was also examined.

Conclusions:

• Variational coupled-cluster methods offer a pathway to explore complex energy landscapes beyond traditional ground-state solutions.
• The choice of reference orbitals significantly impacts the accuracy of VpCCD and pCCD, especially under strong correlation.
• Understanding the energy landscape is crucial for developing robust quantum chemical methods for diverse electronic systems.