Linear Response pCCD-Based Methods: LR-pCCD and LR-pCCD+S Approaches for the Efficient and Reliable Modeling of Excited state Properties

TL;DR

This paper introduces LR-pCCD and LR-pCCD+S methods for efficient and reliable modeling of excited state properties, achieving near-CCSD accuracy with lower computational cost, suitable for simple molecules and polyenes.

Contribution

The work derives new linear response equations for pCCD-based methods and demonstrates their effectiveness in modeling electronic spectra with reduced computational effort.

Findings

LR-pCCD+S accurately reproduces transition dipole moments.

The methods require mean-field-like computational cost.

Effective for molecules with significant double excitation contributions.

Abstract

In this work, we derive working equations for the Linear Response pair Coupled Cluster Doubles (LR-pCCD) ansatz and its extension to singles (S), LR-pCCD+S. These methods allow us to compute electronic excitation energies and transition dipole moments based on a pCCD reference function. We benchmark the LR-pCCD+S model against the {linear response} coupled-cluster singles and doubles method for modeling electronic spectra (excitation energies and transition dipole moments) of the BH, \ce{H2O}, \ce{H2CO}, and furan molecules. We also analyze the effect of orbital optimization within pCCD on the resulting LR-pCCD+S transition dipole moments {and oscillator strengths} and perform a statistical error analysis. We show that the LR-pCCD+S method can correctly reproduce the transition dipole moments features, thus representing a reliable and cost-effective alternative to standard, more expensive electronic structure methods for modeling electronic spectra of simple molecules. Specifically, the proposed models require only mean-field-like computational cost, while excited-state properties may approach the CCSD level of accuracy. Moreover, we demonstrate the capability of our model to simulate electronic transitions with non-negligible contributions of double excitations and the electronic spectra of polyenes of various chain lengths, for which standard electronic structure methods perform purely.