Excitation energies from diffusion Monte Carlo using selected configuration interaction nodes.

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Electronic Structure Theory

Background:

• Quantum Monte Carlo (QMC) excels at ground-state calculations but is underutilized for excited states.
• Accurate excited-state energies are crucial for understanding molecular properties and reactions.

Purpose of the Study:

• To develop and validate a novel Jastrow-free QMC protocol for calculating excited-state energies.
• To assess the accuracy of fixed-node diffusion Monte Carlo (FN-DMC) with selected configuration interaction (sCI) wave functions for excitation energies.

Main Methods:

• Employed a Jastrow-free QMC approach combined with selected configuration interaction (sCI) for nodal surface construction.
• Performed fixed-node diffusion Monte Carlo (FN-DMC) calculations on water and formaldehyde molecules.
• Utilized compact multideterminant expansions and small basis sets (double-ζ).

Main Results:

• Achieved accurate vertical excitation energies for both singlet and triplet states.
• Demonstrated significant cancellation of fixed-node errors between ground and excited states.
• Validated the method on small organic molecules (water, formaldehyde) with all-electron calculations.

Conclusions:

• The proposed Jastrow-free QMC protocol effectively computes accurate excitation energies.
• The sCI-based nodal surfaces in FN-DMC minimize errors for excited-state calculations.
• This method offers a promising avenue for exploring excited-state properties in molecular systems.