Tailoring CIPSI Expansions for QMC Calculations of Electronic Excitations: The Case Study of Thiophene

The perturbatively selected configuration interaction scheme (CIPSI) is particularly effective in constructing determinantal expansions for quantum Monte Carlo (QMC) simulations with Jastrow–Slater wave functions: fast and smooth convergence of ground-state properties balanced descriptions ground excited states different symmetries have been reported. In particular, accurate excitation energies obtained by the pivotal requirement using CIPSI similar second-order perturbation corrections each state, that is, a estimated distance to full limit. Here, we elaborate on selection criterion same symmetry as generating from common orbital set. Using these QMC components functions, compute lowest, bright state thiophene, which challenging due its significant multireference character. resulting vertical are within 0.05 eV best theoretical estimates, already only few thousand determinants. Furthermore, relax ground- excited-state structures following corresponding root variational obtain bond lengths better than 0.01 Å. Therefore, while treatment at level this system quite demanding, QMC, can high-quality structural parameters building affordable relatively few, well-chosen