Excited states of methylene from quantum Monte Carlo

摘要

The ground and lowest three adiabatic excited states of methylene are computed using thevariational Monte Carlo and diffusion Monte Carlo (DMC) methods using progressively largerJastrow–Slater multideterminant complete active space (CAS) wave functions. The highest of thesestates has the same symmetry, ~1A_1, as the first excited state. The DMC excitation energies obtainedusing any of the CAS wave functions are in excellent agreement with experiment, butsingle-determinant wave functions do not yield accurate DMC energies of the states of 1~A_1symmetry, indicating that it is important to include in the wave function Slater determinants thatdescribe static (strong) correlation. Excitation energies obtained using recently proposedpseudopotentials Burkatzki et al., J. Chem. Phys. 126, 234105 (2007) differ from the all-electronexcitation energies by at most 0.04 eV.