arXiv subjects

Massimo Mella

Publications and source records attributed to Massimo Mella.

9 recordsLinked to original sources

arXiv2005

Electronic Quantum Monte Carlo Calculations of Atomic Forces, Vibrations, and Anharmonicities

Atomic forces are calculated for first-row monohydrides and carbon monoxide within electronic quantum Monte Carlo (QMC). Accurate and efficient forces are achieved by using an improved method for moving variational parameters in variational QMC. Newton's method with singular value decomposition (SVD) is combined with steepest descent (SD) updates along directions rejected by the SVD, after initial SD steps. Dissociation energies in variational and diffusion QMC agree well with experiment. The atomic forces agree quantitatively with potential energy surfaces, demonstrating the accuracy of this force procedure. The harmonic vibrational frequencies and anharmonicity constants, derived from the QMC energies and atomic forces, also agree well with experimental values.

physics.chem-ph↗

arXiv2005

Predicting atomic dopant solvation in helium clusters: the MgHe$_n$ case

We present a quantum Monte Carlo study of the solvation and spectroscopic properties of the Mg doped helium clusters MgHe$_n$ with $n=2-50$. Three high level (MP4, CCSD(T) and CCSDT) MgHe interaction potentials have been used to study the sensitivity of the dopant location on the shape of the pair interaction. Despite the similar MgHe well depth, the pair distribution functions obtained in the diffusion Monte Carlo simulations markedly differ for the three pair potentials, therefore indicating different solubility properties for Mg in He$_n$. Moreover, we found interesting size effects for the behavior of the Mg impurity. As a sensitive probe of the solvation properties, the Mg excitation spectra have been simulated for various cluster sizes and compared with the available experimental results. The interaction between the excited $^1$P Mg atom and the He moiety has been approximated using the Diatomics-in-Molecules method and the two excited $^1Π$ and $^1Σ$ MgHe potentials. The shape of the simulated MgHe$_{50}$ spectra show a substantial dependency on the location of the Mg impurity, and hence on the MgHe pair interaction employed. To unravel the dependency of the solvation behavior on the shape of the computed potentials, exact Density Functional Theory has been adapted to the case of doped He$_n$ and various energy distributions have been computed. The results indicate the shape of the repulsive part of the MgHe potential as an important cause of the different behaviours.

physics.atm-clus↗

arXiv2001

Ground state and excitation dynamics in Ag doped helium clusters

We present a quantum Monte Carlo study of the structure and energetics of silver doped helium clusters AgHe$_n$ for $n$ up to 100. Our simulations show the first solvation shell of the Ag atom to be composed by roughly 20 He atoms, and to possess a structured angular distribution. Moreover, the electronic $^2$P$_{1/2}\leftarrow ^2$S$_{1/2}$ and $^2$P$_{3/2}\leftarrow ^2$S$_{1/2}$ electronic transitions of the embedded silver impurity have been studied as a function of the number ofhelium atoms. The computed spectra show a redshift for $n\leq 15$ and an increasing blueshift for larger clusters, a feature attributed to the effect of the second solvation shell of He atoms. For the largest cluster, the computed excitation spectrum is found in excellent agreement with the ones recorded in superfluid He clusters and bulk. No signature of the direct formation of proposed AgHe$_2$ exciplex is present in the computed spectra of AgHe$_{100}$.

physics.atm-clus↗

arXiv2001

o-Positronium scattering off H and He

Exploiting an approach similar to the R-matrix theory, the diffusion Monte Carlo method is employed to compute phase shifts and threshold cross sections for the elastic scattering of o-positronium off light atoms. Results are obtained for Ps-H and Ps-He as representative cases of open and closed shell targets. The method allows for an exact treatment of both correlation and exchange interactions, and represents the most promising approach to deal with these effects in more complicated targets. In particular the Ps-He threshold cross section, computed in a many body framework for the first time, represents a standard by which past and future numerical and experimental estimates can be judged.

physics.atom-ph↗

arXiv2001

Annihilation rate in positronic systems by quantum Monte Carlo. e$^+$LiH as test case

An accurate method to compute the annihilation rate in positronic systems by means of quantum Monte Carlo simulations is tested and compared with previously proposed methods using simple model systems. This method can be applied within all the quantum Monte Carlo techniques, just requiring to accumulate the positron-electron distribution function. The annihilation rate of e$^+$LiH as a function of the internuclear distance is studied using a model potential approach to eliminate the core electrons of Li, and explicitly correlated wave functions to deal with all the remaining particles. These results allow us to compute vibrationally averaged annihilation rates, and to understand the effect of the Li$^+$ electric field on positron and electron distributions.

physics.chem-ph↗

arXiv2001

Robust wave function optimization procedures in quantum Monte Carlo methods

The energy variance optimization algorithm over a fixed ensemble of configurations in variational Monte Carlo is formally identical to a problem of fitting data: we reexamine it from a statistical maximum-likelihood point of view. We detect the origin of the problem of convergence that is often encountered in practice and propose an alternative procedure for optimization of trial wave functions in quantum Monte Carlo. We successfully test this proposal by optimizing a trial wave function for the Helium trimer.

physics.atm-clus↗

arXiv2000

Explicitly correlated trial wave functions in Quantum Monte Carlo calculations of excited states of Be and Be-

We present a new form of explicitly correlated wave function whose parameters are mainly linear, to circumvent the problem of the optimization of a large number of non-linear parameters usually encountered with basis sets of explicitly correlated wave functions. With this trial wave function we succeeded in minimizing the energy instead of the variance of the local energy, as is more common in quantum Monte Carlo methods. We applied this wave function to the calculation of the energies of Be 3P (1s22p2) and Be- 4So (1s22p3) by variational and diffusion Monte Carlo methods. The results compare favorably with those obtained by different types of explicitly correlated trial wave functions already described in the literature. The energies obtained are improved with respect to the best variational ones found in literature, and within one standard deviation from the estimated non-relativistic limits

physics.atom-ph↗

arXiv2000

Stability and production of positron-diatomic molecule complexes

The energies at geometries close to the equilibrium for the e$^+$BeO and e$^+$LiF ground states were computed by means of diffusion Monte Carlo simulations. These results allow us to predict the equilibrium geometries and the vibrational frequencies for these exotic systems,and to discuss their stability with respect to the various dissociation channels. Since the adiabatic positron affinities were found to be smaller than the dissociation energies for both complexes, we propose these two molecules as possible candidates in the challenge to produce and detect stable positron-molecule systems.

physics.atom-ph↗

arXiv2000

A variational Monte Carlo calculation of dynamic multipole polarizabilities and van der Waals coefficients of the PsH system

The first three dynamic multipole polarizabilities for the ground state of hydrogen, helium, hydride ion, and positronium hydride PsH have been computed using the variational Monte Carlo (VMC) method and explicitly correlated wave functions. Results for the static dipole polarizability by means of the diffusion Monte Carlo (DMC) method and the finite field approach show the VMC results to be quite accurate. From these dynamic polarizabilities van der Waals dispersion coefficients for the interaction of PsH with ordinary electronic systems can be computed, allowing one to predict the dispersion energy for the interaction between PsH and less exotic atoms and molecules.

physics.atom-ph↗