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Unitary coupled-cluster based self-consistent polarization propagator theory: A third-order formulation and pilot
Junzi Liu1, Ayush Asthana1, Lan Cheng1
1Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, USA.
The Journal of Chemical Physics
|July 2, 2018
Summary
A new computational method, Unitary Coupled-Cluster (UCC) based third-order self-consistent polarization propagator theory (UCC3), accurately calculates excitation energies for molecules. This method shows promise for studying electronically excited states.
Area of Science:
- Quantum Chemistry
- Computational Chemistry
- Theoretical Chemistry
Background:
- Accurate calculation of excitation energies is crucial for understanding molecular electronic structure and photochemistry.
- Existing methods like Algebraic Diagrammatic Construction (ADC) and Equation-of-Motion Coupled-Cluster (EOM-CC) have limitations in accuracy and applicability.
Purpose of the Study:
- To develop a novel, accurate, and efficient computational method for calculating electronic excitation energies.
- To introduce the third-order self-consistent polarization propagator method based on Unitary Coupled-Cluster (UCC) parametrization, termed UCC3.
- To provide a robust theoretical framework for non-perturbative treatment of excited electronic states.
Main Methods:
- Development of the UCC3 method utilizing a unitary coupled-cluster (UCC) parametrization of the ground-state wavefunction.
- Employing unitary-transformed excitation operators for the excitation manifold.
- Derivation of UCC3 working equations using an expansion for the UCC transformed Hamiltonian involving Bernoulli numbers.
Main Results:
- The UCC3 method provides excitation energies accurate to the third order, particularly for excited states dominated by single excitations.
- Comparison with benchmark full configuration interaction (FCI) values for H2O, HF, N2, Ne, CH2, BH, and C2 molecules demonstrates the accuracy of UCC3.
- UCC-based polarization propagator theory offers an alternative derivation for the strict third-order algebraic diagrammatic construction [ADC(3)-s] method.
Conclusions:
- The UCC3 method is a highly accurate approach for calculating excitation energies.
- UCC-based self-consistent polarization propagator theory presents a promising avenue for developing non-perturbative, Hermitian formulations for electronically excited states.
- This work establishes a new theoretical framework with significant potential for advancing computational quantum chemistry.
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