Massimo Mella

Positronium chemistry by quantum Monte Carlo. I. Positronium-first row atom complexes

The stability of the ground and excited states of Positronium–atom complexes [A, Ps], Ps =[e+,e−], has been explored for A=Li, B, C, O, F using variational and diffusion Monte Carlo techniques. From the numerical results of our simulations it turns out that the ground state of the complexes [Li, Ps]2S, [C, Ps]3S, [O, Ps]1P, and [F, Ps]2S is stable against the break up in the two neutral fragments A and Ps, while the ground state of [B, Ps]2P has an energy above the same dissociation threshold. As to the excited states, the only possible candidate, [F, Ps]2P, has a total energy statistically equal to the lower dissociation threshold, i.e. it does not seem to be stable against the dissociation.

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 often encounters problems of convergence. Being formally identical to a problem of fitting data, we re-examine it from a statistical maximum-likelihood point of view. We show that the assumption of an underlying Gaussian distribution of the local energy, implicit in the standard variance minimization scheme, is not theoretically nor practically justified, and frequently generates convergence problems. We propose alternative procedures for optimization of trial wave functions in quantum Monte Carlo and successfully test them by optimizing a trial wave function for the helium trimer.

Quantum Monte Carlo investigation of small 4He clusters with a 3He impurity

Small helium (4He) clusters containing the lighter isotope 3He are studied by means of quantum Monte Carlo methods. Accurate ground state energies and structural properties are obtained using accurate trial wave functions and the Tang–Tonnies–Yiu (TTY) helium–helium pair potential. The dimer 4He–3He is not bound; as well as the trimer 4He3He2. The smallest cluster containing 3He is 4He2 3He with a nonrigid structure having a marked linear contribution. Interestingly, this weakly bound system, with an energy one order of magnitude less than the 4He3 trimer, is able to bind another 3He atom, forming the tetramer 4He2 3He2, which shows the odd feature of having five out of six unbound pairs. In general, the substitution of a single 4He atom in a pure cluster with a 3He atom leads to an energetic destabilization, as the pair 4He–3He is not bound. The isotopic impurity is found to perturb only weakly the distributions of the remaining 4He atoms, which retain the high floppiness already found in the pure cluste...

Positron chemistry by quantum Monte Carlo. II. Ground-state of positron-polar molecule complexes

@LiF,e 1 # 2,1 S 1 is stable against the dissociation either in the two fragments M and e 1 or in the other two fragments M 1 and Ps5@e 1 ,e 2 #, while the ground-state of @H2O,e 1 # 2,1 A1, and of @HF,e 1 # 2,1 S 1 has an energy equal to the dissociation threshold, M and e 1 . We also compare the predicted vertical positron affinity ~PA! with high quality vertical electron affinity ~EA! and discuss the relevant difference between the two values.

Modulating antimicrobial activity by synthesis: dendritic copolymers based on nonquaternized 2-(dimethylamino)ethyl methacrylate by Cu-mediated ATRP.

The synthesis of novel star-like heteroarms polymers A(BC)(n) containing m-PEG (block A), methylmethacrylate (MMA), and nonquaternized 2-(dimethylamino)ethyl methacrylate (DMAEMA) (blocks BC) is here reported. We demonstrated that copolymer films with comparable amounts of DMAEMA have antimicrobial properties strongly depending on the topological structure (i.e., the number of arms) of the composing copolymers. We interpret the highest antimicrobial activity of A(BC)(2) with respect to A(BC)(4) and linear copolymers (respectively, A(BC)(2) ≥ A(BC)(4) > A(BC)) as probably due to the formation of strong hydrogen bonds between close amino-ammonium groups in the A(BC)(2) film. Strong hydrogen bonds seem to be somewhat disfavored in the case of the linear species by the difference in both polymer architecture and film morphology compared with the A(BC)(2) and A(BC)(4) architectures.

Computing accurate forces in quantum Monte Carlo using Pulay's corrections and energy minimization

In order to overcome the difficulty of optimizing molecular geometry using quantum Monte Carlo methods, we introduce various approximations to the exact force expectation value. We follow Pulay’s suggestion [Mol. Phys. 17, 153 (1969)] to correct the Hellmann–Feynman estimator by introducing the contributions due to the changes in the wave function with respect to the nuclear positions. When used in conjunction with energy-optimized explicitly correlated trial wave functions for H2 and LiH, these approximations appear to yield accurate forces using both the variational and diffusion Monte Carlo methods. Also, the accuracy of the second-order estimate of the Hellmann–Feynman force estimator was investigated employing our energy-optimized trial wave functions, and an erratic behavior was uncovered for some of the studied bond lengths. The additional computational cost required to compute the corrections to the Hellmann–Feynman estimator was found to be only a small fraction of the cost for a simple mean ener...

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 AgHen for n up to 100. Our simulations show the first solvation shell of the Ag atom to include roughly 20 He atoms, and to possess a structured angular distribution. Moreover, the 2P1/2←2S1/2 and 2P3/2←2S1/2 electronic transitions of the embedded silver impurity have been studied as a function of the number of helium atoms. The computed spectra show a redshift for n⩽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 the proposed AgHe2 exciplex is present in the computed spectrum of AgHe100. To explain the absence of the fluorescent D2 line in the experiments, a relaxation mechanism between the 2P3/2 and the 2P1/2 states is proposed on the b...

Positron and positronium chemistry by quantum Monte Carlo. IV. Can this method accurately compute observables beyond energy

Many different properties of the positron containing systems PsH, [Li,e+], LiPs, and [LiH,e+], were computed using both variational Monte Carlo and fixed node diffusion Monte Carlo methods, and explicitly correlated trial wave functions. Our results show that these techniques can accurately compute not only energy values, but also other observables. Our 〈δ(r+−)〉 values for PsH, [Li,e+], and LiPs are in good agreement with the most recent state of the art correlated calculations, while for [LiH,e+] our calculations are the first to give reliable results.

Predicting atomic dopant solvation in helium clusters: The MgHen 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 1P Mg atom and the He moiety has been approximated using the diatomics-in-molecules method and the two excited 1pi and 1sigma MgHe potentials. The shape of the simulated MgHe50 spectra shows 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 behaviors.

Intermolecular forces and fixed-node diffusion Monte Carlo: A brute force test of accuracies for He2 and He–LiH

The accuracy of the fixed-node approximation and diffusion Monte Carlo method in computing the interaction energy of van der Waals systems was investigated. Tests were carried out by simulating the electronic structures of He2 and He–LiH. These two systems were chosen as representative of two fundamentally different interactions, namely the weak dispersion forces in He2 and the dipole/induced–dipole interaction in He–LiH. The results for both systems are in excellent agreement with “state of the art” calculations, thereby indicating a high accuracy for the fixed-node approximation. Also, our interaction energies for He–LiH indicate that the coupled cluster single double triple method [Taylor and Hinde, J. Chem. Phys. 111, 973 (1999)] gives an accurate prediction of the interaction potential for that system.