Multiple-Resonance Local Wave Functions for Accurate Excited States in Quantum Monte Carlo

We
introduce a novel class of local multideterminant Jastrow–Slater
wave functions for the efficient and accurate treatment of excited
states in quantum Monte Carlo. The wave function is expanded as a
linear combination of excitations built from multiple sets of localized
orbitals that correspond to the bonding patterns of the different
Lewis resonance structures of the molecule. We capitalize on the concept
of orbital domains of local coupled-cluster methods, which is here
applied to the active space to select the orbitals to correlate and
construct the important transitions. The excitations are further grouped
into classes, which are ordered in importance and can be systematically
included in the Jastrow–Slater wave function to ensure a balanced
description of all states of interest. We assess the performance of
the proposed wave function in the calculation of vertical excitation
energies and excited-state geometry optimization of retinal models
whose π → π* state has a strong intramolecular
charge-transfer character. We find that our multiresonance wave functions
recover the reference values of the total energies of the ground and
excited states with only a small number of excitations and that the
same expansion can be flexibly used at very different geometries.
Furthermore, significant computational saving can also be gained in
the orbital optimization step by selectively mixing occupied and virtual
orbitals based on spatial considerations without loss of accuracy
on the excitation energy. Our multiresonance wave functions are therefore
compact, accurate, and very promising for the calculation of multiple
excited states of different character in large molecules.