title: How large are nonadiabatic effects in atomic and diatomic systems?
abstract: With recent developments in simulating nonadiabatic systems to high accuracy, it has become possible to determine how much energy is attributed to nuclear quantum effects beyond zero-point energy. In this work, we calculate the non-relativistic ground-state energies of atomic and molecular systems without the Born-Oppenheimer approximation. For this purpose, we utilize the fixed-node diffusion Monte Carlo method, in which the nodes depend on both the electronic and ionic positions. We report ground-state energies for all systems studied, ionization energies for the first-row atoms and atomization energies for the first-row hydrides. We find the ionization energies of the atoms to be nearly independent of the Born-Oppenheimer approximation, within the accuracy of our results. The atomization energies of molecular systems, however, show small effects of the nonadiabatic coupling between electrons and nuclei.
authors.affiliation: Department of Physics, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Yubo
authors.initials: Y
authors.lastname: Yang
authors.affiliation: Department of Physics, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Ilkka
authors.initials: I
authors.lastname: Kylänpää
authors.affiliation: Department of Physics, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Norm M
authors.initials: NM
authors.lastname: Tubman
authors.affiliation: Oak Ridge National Laboratory, Materials Sciences & Technology Division, Oak Ridge, Tennessee 37831, USA.
authors.forename: Jaron T
authors.initials: JT
authors.lastname: Krogel
authors.affiliation: Department of Chemistry, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Sharon
authors.initials: S
authors.lastname: Hammes-Schiffer
authors.affiliation: Department of Physics, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: David M
authors.initials: DM
authors.lastname: Ceperley