Combining Effective Hamiltonians and Brillouin-Wigner Approach: A Perturbative Approach to Spectroscopy

TL;DR

This paper introduces a multi-step perturbative method combining effective Hamiltonians and Brillouin-Wigner corrections to efficiently compute molecular electronic spectra.

Contribution

It presents a novel multi-step perturbative scheme integrating effective Hamiltonians with Brillouin-Wigner corrections for spectroscopy.

Findings

Demonstrates robustness and efficiency on model Hamiltonians

Applicable to electronic spectroscopy of molecular systems

Provides accurate energy estimates with reduced computational cost

Abstract

The numerical cost of variational methods suggests using perturbative approaches to determine the electronic structure of molecular systems. In this work, a sequential construction of effective Hamiltonians drives the definition of approximate model functions and energies in a multi-state Rayleigh-Schr\"odinger perturbative scheme. A second step takes advantage of an updated partitioning of the Hamiltonian to perform a state-specific Brillouin-Wigner energy correction based on a well-tempered perturbation expansion. The multi-step RSBW method is exemplified on model-Hamiltonians to stress its robustness, efficiency and applicability to spectroscopy determination.