Approaching an exact treatment of electronic correlations at solid surfaces: The binding energy of the lowest bound state of helium adsorbed on MgO(100)

Abstract

In this work we employ ab initio electronic structure theory at a very high level to resolve a long standing experimental controversy; the interaction between helium and the MgO (100) surface has been studied extensively by other groups, employing diverse experimental approaches. Nevertheless, the binding energy of the lowest bound state is still unclear: the existence of a state at around −5.5 meV is well established but a state at −10 meV has also been reported. The MgO (100)-He system captures the fundamental physics involved in many adsorption problems; the weak binding is governed by long-range electronic correlation for which a fully predictive theory applicable to the solid state has been elusive. The above-mentioned experimental controversy can now be resolved on the basis of the calculations presented in this work. We performed three-dimensional vibrational dynamics calculations on a highly accurate potential-energy surface. The latter was constructed using a method which systematically approaches the exact limit in its treatment of electronic correlation. The outcome is clear: our calculations do not support the existence of a bound state around −10 meV. The interaction between molecules and crystalline surfaces is of great fundamental and technological interest and is extensively studied both experimentally and theoretically [1][2][3]. One particular example is physisorption and scattering of helium atoms on oxide surfaces. In the last two decades the latter process, employing molecular-beam techniques, has been developed into a powerful tool for the analysis of surface structure and dynamics [4][5][6][7][8]. It is particularly useful for studying insulating surfaces, such as MgO [4,9]. At close proximity the helium-surface interaction potential is dominated by the exponentially growing corrugated repulsive wall, which is a consequence of the mutual exchange repulsion between the helium and surface electron densities. The repulsive potential is two-dimensional (2D) corrugated and leads to a complicated diffraction pattern for helium scattered from the surface. In the range of intermediate He-surface distances, there exists a very shallow attractive potential due to the weak van der Waals interactions, which drops off with distance from the surface as 1/z 3 [10]. Such a potential can support several bound states (in fact several 2D bands of bound states), which correspond to vibrational levels of helium atoms physisorbed on the surface.The delicacy of the balance between different components of the helium-surface interaction makes quantitatively accurate theoretical predictions of the behavior of helium atoms on the surface extremely difficult. Standard density functional theory (DFT), which is the common tool in solid-state simulations (i.e., local-density approximation, generalized gradient approximation, or hybrids), does not capture van der Waals dispersion. In principle it can be used for calculating * r.martinezcasado@imperial.ac.uk † denis.usvyat@chemie.uni-regensburg.de the repulsive wall at th...

Diffraction of helium on MgO(100) surface calculated from first-principles

“…Interestingly, the quite moderate value of 1.07 for the intra-monomer correction factor indicates that in this particular component the method error of LMP2 and the basis set incompleteness error (of the AVTZ basis as used in the periodic calculation) compensate each other to a large extent when referenced to the CCSD(T)/basis-set-extrapolated value. However, as was recently demonstrated 21 for a single point CCSDT(Q) calculation on a small He-Mg 2 O 2 cluster, the CCSD(T) method is not yet a converged reference for this system with respect to the order of the correlated method (at least at the scale of the interaction energies). Therefore the upscaling factors corresponding to the CCSD(T)/basis-set-extrapolated results do not provide sufficient accuracy.…”

“…For further reference we denote it as AVTZ. This basis is sufficiently large to reach saturation at the Hartree-Fock (HF) level, 21 but not yet for the correlation energy contribution. Hence, the remaining basis set incompleteness error is corrected together with above mentioned method error correction scheme, as described in detail below.…”

“…Hence, in order to extrapolate the core correlation contribution for an arbitrary site we applied for it the 2D-sine law model. 20 It has recently been established, 20,21,34 that the MP2 level of theory significantly underestimates the adsorption energy of noble gasses on the MgO surface, and especially so for He. Moreover, this underestimation is further magnified by the basis set deficiency error, which has the same sign as the MP2 method error for these systems.…”

“…Similar computational protocols have already been applied in earlier work to study the geometrical frustration of an argon monolayer adsorbed on MgO(100) 20 and to determine the lowest bound state of helium adsorbed on MgO(100). 21 Scattering simulations are then performed for various energies, polar angles, and two different azimuthal angles of the incident beam, for which high quality experimental scattering data is available.…”

Range-separated double-hybrid density-functional theory applied to periodic systems

“…[34][35][36][37][38][39][40][41][42][43][44][45] The adaptation of the local MP2 method to periodic systems has been done in the last decade, 18,46,47 and over the years has been successfully applied to quite a rich variety of systems. [48][49][50][51][52][53][54][55][56][57][58][59][60] The paper is structured as follows. In Sec.…”