Patrick Huang

Quantum solvation and molecular rotations in superfluid helium clusters

Spectroscopic experiments on molecules embedded in free clusters of liquid helium reveal a number of unusual features deriving from the unique quantum behavior of this nanoscale matrix environment. The apparent free rotation of small molecules in bosonic 4He clusters is one of the experimentally most well documented of these features. In this Focus article, we set this phenomenon in the context of experimental and theoretical advances in this field over the last ten years, and describe the microscopic insight which it has provided into the nature and dynamic consequences of quantum solvation in a superfluid. We provide a comprehensive theoretical analysis which is based on a unification of conclusions drawn from diffusion and path integral Monte Carlo calculations. These microscopic quantum calculations elucidate the origin of the empirical free rotor spectrum, and its relation to the boson character and superfluid nature of the quantum nanosolvent. The free rotor behavior of the molecular rotation is pre...

Self-consistent embedding theory for locally correlated configuration interaction wave functions in condensed matter

We present new developments on a density-based embedding strategy for the electronic structure of localized feature in periodic, metallic systems [see T. Kluner et al., J. Chem. Phys. 116, 42 (2002), and references therein]. The total system is decomposed into an embedded cluster and a background, where the background density is regarded as fixed. Its effect on the embedded cluster is modeled as a one-electron potential derived from density functional theory. We first discuss details on the evaluation of the various contributions to the embedding potential and provide a strategy to incorporate the use of ultrasoft pseudopotentials in a consistent fashion. The embedding potential is obtained self-consistently with respect to both the total and embedded cluster densities in the embedding region, within the framework of a frozen background density. A strategy for accomplishing this self-consistency in a numerically stable manner is presented. Finally, we demonstrate how dynamical correlation effects can be treated within this embedding framework via the multireference singles and doubles configuration interaction method. Two applications of the embedding theory are presented. The first example considers a Cu dimer embedded in the (111) surface of Cu, where we explore the effects of different models for the kinetic energy potential. We find that the embedded Cu density is reasonably well-described using simple models for the kinetic energy. The second, more challenging example involves the adsorption of Co on the (111) surface of Cu, which has been probed experimentally with scanning tunneling microscopy [H. C. Manoharan et al., Nature (London) 403, 512 (2000)]. In contrast to Kohn-Sham density functional theory, our embedding approach predicts the correct spin-compensated ground state.

Localized helium excitations in 4 He N -benzene clusters

We compute ground and excited state properties of small helium clusters

Effects of molecular rotation on densities in doped 4He clusters

{}^{4}{\mathrm{He}}_{N}

Multiple solvation configurations around phthalocyanine in helium droplets.

containing a single benzene impurity molecule. Ground-state structures and energies are obtained for

Ab initio explanation of tunneling line shapes for the kondo impurity state.

N=1, 2, 3, 14

Structure and Energetics of Helium Adsorption on Nanosurfaces

from an importance-sampled, rigid-body diffusion Monte Carlo method. Excited state energies due to helium vibrational motion near the molecule surface are evaluated using the projection operator, imaginary time spectral evolution method. We find excitation energies of up to

THE FINITE-TEMPERATURE PATH INTEGRAL MONTE CARLO METHOD AND ITS APPLICATION TO SUPERFLUID HELIUM CLUSTERS

\ensuremath{\sim}23\mathrm{K}

Density dependence of the hydrodynamic response to SF6 rotation in superfluid helium

above the ground state. These states all possess vibrational character of helium atoms in a highly anisotropic potential due to the aromatic molecule, and can be categorized in terms of localized and collective vibrational modes. These results appear to provide precursors for a transition from localized to collective helium excitations at molecular nanosubstrates of increasing size. We discuss the implications of these results for analysis of anomalous spectral features in recent spectroscopic studies of large aromatic molecules in helium clusters.

Large-Scale First-Principles Molecular Dynamics Simulations with Electrostatic Embedding: Application to Acetylcholinesterase Catalysis

The effects of including rotational degrees of freedom on helium solvation densities in molecule-doped helium clusters are investigated for a variety of molecules. Helium densities and cluster energetics are calculated with diffusion Monte Carlo methods. The rotationally induced changes in the helium density distributions are examined and quantified with a theoretical estimator applicable to molecules of arbitrary symmetry. This analysis leads to a discussion of adiabatic following of molecular rotation in a solvating helium environment. We make a detailed comparative study of the effect of molecular rotation as a function of four impurity molecules with varying mass and symmetry: SF6, OCS, HCN, and benzene (C6H6). We find that even for the heaviest rotors, only a fraction of the solvating helium density adiabatically follows the molecular motion in the quantum ground state. For the lightest molecule, HCN, a negligible degree of adiabatic following is found. A discussion of the various definitions is pres...