arXiv subjects

Saverio Moroni

Publications and source records attributed to Saverio Moroni.

48 records · Page 3Linked to original sources

arXiv2010

Size-consistent variational approaches to non-local pseudopotentials: standard and lattice regularized diffusion Monte Carlo methods revisited

We propose improved versions of the standard diffusion Monte Carlo (DMC) and the lattice regularized diffusion Monte Carlo (LRDMC) algorithms. For the DMC method, we refine a scheme recently devised to treat non-local pseudopotential in a variational way. We show that such scheme --when applied to large enough systems-- maintains its effectiveness only at correspondingly small enough time-steps, and we present two simple upgrades of the method which guarantee the variational property in a size-consistent manner. For the LRDMC method, which is size-consistent and variational by construction, we enhance the computational efficiency by introducing (i) an improved definition of the effective lattice Hamiltonian which remains size-consistent and entails a small lattice-space error with a known leading term, and (ii) a new randomization method for the positions of the lattice knots which requires a single lattice-space.

cond-mat.other↗

arXiv2009

Zero-temperature dynamics of solid 4He from quantum Monte Carlo simulations

The lattice dynamics of hcp crystalline 4He is studied at zero temperature and for two different densities (near and far from melting), using a ground-state path-integral quantum Monte Carlo technique. The complete phonon dispersion is obtained, with particular attention to the separation of optic and acoustic branches and to the identification of transverse modes. Our study also sheds light onto the residual coherence affecting quasiparticle excitations in the intermediate momentum region, in-between the phonon and nearly-free particle regimes.

cond-mat.other↗

arXiv2006

Local-spin-density functional for multideterminant density functional theory

Based on exact limits and quantum Monte Carlo simulations, we obtain, at any density and spin polarization, an accurate estimate for the energy of a modified homogeneous electron gas where electrons repel each other only with a long-range coulombic tail. This allows us to construct an analytic local-spin-density exchange-correlation functional appropriate to new, multideterminantal versions of the density functional theory, where quantum chemistry and approximate exchange-correlation functionals are combined to optimally describe both long- and short-range electron correlations.

cond-mat.mtrl-sci↗

arXiv2005

Computer simulation of quantum melting in hydrogen clusters

We introduce a new criterion--based on multipole dynamical correlations calculated within Reptation Quantum Monte Carlo--to discriminate between a melting vs. freezing behavior in quantum clusters. This criterion is applied to small clusters of para-hydrogen molecules (both pristine and doped with a CO cromophore), for cluster sizes around 12 molecules. This is a magic size at which para-hydrogen clusters display an icosahedral structure and a large stability. In spite of the similar geometric structure of CO@(pH2)_12 and (pH2)_13, the first system has a rigid, crystalline, behavior, while the second behaves more like a superfluid (or, possibly, a supersolid).

cond-mat.other↗

arXiv2004

A local density functional for the short-range part of the electron-electron interaction

Motivated by recent suggestions --to split the electron-electron interaction into a short-range part, to be treated within the density functional theory, and a long-range part, to be handled by other techniques-- we compute, with a diffusion Monte Carlo method, the ground-state energy of a uniform electron gas with a modified, short-range-only electron-electron interaction $\erfc(μr)/r$, for different values of the cutoff parameter $μ$ and of the electron density. After deriving some exact limits, we propose an analytic representation of the correlation energy which accurately fits our Monte Carlo data and also includes, by construction, these exact limits, thus providing a reliable ``short-range local-density functional''.

cond-mat.mtrl-sci↗

arXiv2004

Pair-distribution functions of the two-dimensional electron gas

Based on its known exact properties and a new set of extensive fixed-node reptation quantum Monte Carlo simulations (both with and without backflow correlations, which in this case turn out to yield negligible improvements), we propose a new analytical representation of (i) the spin-summed pair-distribution function and (ii) the spin-resolved potential energy of the ideal two-dimensional interacting electron gas for a wide range of electron densities and spin polarization, plus (iii) the spin-resolved pair-distribution function of the unpolarized gas. These formulae provide an accurate reference for quantities previously not available in analytic form, and may be relevant to semiconductor heterostructures, metal-insulator transitions and quantum dots both directly, in terms of phase diagram and spin susceptibility, and indirectly, as key ingredients for the construction of new two-dimensional spin density functionals, beyond the local approximation.

cond-mat.str-el↗

arXiv2003

Rotational dynamics of CO solvated in small He clusters: a quantum Monte Carlo study

The rotational dynamics of CO single molecules solvated in small He clusters (CO@He_N) has been studied using Reptation Quantum Monte Carlo for cluster sizes up to N=30. Our results are in good agreement with the roto-vibrational features of the infrared spectrum recently determined for this system, and provide a deep insight into the relation between the structure of the cluster and its dynamics. Simulations for large N also provide a prediction of the effective moment of inertia of CO in the He nano-droplet regime, which has not been measured so far.

cond-mat↗

arXiv2003

Two-dimensional electron gas: correlation energy versus density and spin polarization

We propose a simple analytic representation of the correlation energy for the two-dimensional electron gas, as a function of the density and the spin polarization. This new parametrization includes most of the known high- and low- density limits and fits our new fixed-node diffusion Monte Carlo simulations, performed for a wide range of electron densities and spin-polarization states. In this way we provide a reliable local-spin-density energy functional for two-dimensional systems. The corresponding correlation potential is discussed and compared with previous models.

cond-mat.str-el↗

arXiv2003

Correlation energy and spin polarization in the 2D electron gas

The ground state energy of the two--dimensional uniform electron gas has been calculated with fixed--node diffusion Monte Carlo, including backflow correlations, for a wide range of electron densities as a function of spin polarization. We give a simple analytic representation of the correlation energy which fits the density and polarization dependence of the simulation data and includes several known high- and low-density limits. This parametrization provides a reliable local spin density energy functional for two-dimensional systems and an estimate for the spin susceptibility. Within the proposed model for the correlation energy, a weakly first--order polarization transition occurs shortly before Wigner crystallization as the density is lowered.

cond-mat.str-el↗

arXiv2002

Structure, rotational dynamics, and superfluidity of small OCS-doped He clusters

The structural and dynamical properties of OCS molecules solvated in Helium clusters are studied using reptation quantum Monte Carlo, for cluster sizes n=3-20 He atoms. Computer simulations allow us to establish a relation between the rotational spectrum of the solvated molecule and the structure of the He solvent, and of both with the onset of superfluidity. Our results agree with a recent spectroscopic study of this system, and provide a more complex and detailed microscopic picture of this system than inferred from experiments.

cond-mat↗

arXiv1996

Momentum distribution of liquid helium

We have obtained the one--body density matrix and the momentum distribution $n(p)$ of liquid $^4$He at $T=3D0^o$K from Diffusion Monte Carlo (DMC) simulations, using trial functions optimized via the Euler Monte Carlo (EMC) method. We find a condensate fraction smaller than in previous calculations. Though we do not explicitly include long--range correlations in our calculations, we get a momentum distribution at long wavelength which is compatible with the presence of long--range correlations in the exact wave function. We have also studied $^3$He, using fixed--node DMC, with nodes and trial functions provided by the EMC. In particular, we analyze the momentum distribution $n(p)$ with respect to the discontinuity $Z$ as well as the singular behavior, at the Fermi surface. We also show that an approximate factorization of the one-body density matrix $ρ(r)\simeq ρ_0(r)ρ_B(r)$ holds, with $ρ_0(r)$ and $ρ_B(r)$ respectively the density matrix of the ideal Fermi gas and the density matrix of a Bose $^3$He.

cond-mat↗

arXiv1996

Density-functional theory of freezing of quantum liquids at zero temperature using exact liquid-state linear response

We apply density functional theory to study the freezing of superfluid {$^{4}\rm{He}$}, charged bosons and charged fermions at zero temperature. We employ accurate Quantum Monte Carlo data for the linear response function in the uniform phase of these systems, a quantity that has different behavior for large values of the wavevector than previously assumed. We find that, as a result of this {\it{exact}} behavior, different approximations in the density functional theory of freezing that involve linear response, all fail to correctly describe the crystallization in {\it{three dimensions}}, while yielding satisfactory predictions in {\it{two dimensions}}. This demonstrates the shortcomings of the currently popular density functional approximate theories to describe $3d$-freezing in the quantum regime. We also investigate the consequences of the exact asymptotic behavior of response functions on the form of effective interactions and polarization potentials in the electron gas, at small distances.

cond-mat↗