## Publications

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**ABSTRACT:**Whereas most of the phenomena associated with superfluidity have been observed in finite-size helium systems, the nucleation of quantized vortices has proven elusive. Here we show using time-dependent density functional simulations that the solvation of a Ba(+) ion created by photoionization of neutral Ba at the surface of a (4)He nanodroplet leads to the nucleation of a quantized ring vortex. The vortex is nucleated on a 10 ps timescale at the equator of a solid-like solvation structure that forms around the Ba(+) ion. The process is expected to be quite general and very efficient under standard experimental conditions.The Journal of Chemical Physics 04/2014; 140(13):131101. · 3.12 Impact Factor -
##### Article: Infinite swapping in curved spaces.

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**ABSTRACT:**We develop an extension of the infinite swapping and partial infinite swapping techniques [N. Plattner, J. D. Doll, P. Dupuis, H. Wang, Y. Liu, and J. E. Gubernatis, J. Chem. Phys. 135, 134111 (2011)] to curved spaces. Furthermore, we test the performance of infinite swapping and partial infinite swapping in a series of flat spaces characterized by the same potential energy surface model. We develop a second order variational algorithm for general curved spaces without the extended Lagrangian formalism to include holonomic constraints. We test the new methods by carrying out NVT classical ensemble simulations on a set of multidimensional toroids mapped by stereographic projections and characterized by a potential energy surface built from a linear combination of decoupled double wells shaped purposely to create rare events over a range of temperatures.The Journal of Chemical Physics 01/2014; 140(1):014103. · 3.12 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**The structure and energetics of exciplexes M*((2)L)Hen (M = Cu, Ag and Au; L = P and D) in their vibrational ground state are studied by employing diffusion Monte Carlo (DMC). Interaction potentials between the excited coinage metals and He atoms are built using the Diatomics-in-Molecule (DIM) approach and ab initio potential curves for the M((2)L)-He dimers. Extending our previous work [Cargnoni et al., J. Phys. Chem. A, 2011, 115, 7141], we computed the dimer potential for Au in the (2)P and (2)D states, as well for Cu and Ag in the (2)D state, employing basis set superposition error-corrected Configuration Interaction calculations. We found that the (2)Π potential correlating with the (2)P state of Au is substantially less binding than for Ag and Cu, a trend well supported by the M(+) ionic radiuses. Conversely, the interaction potentials between a (n - 1)d(9)ns(2 2)D metal and He present a very weak dependency on M itself or the projection of the angular momentum along the dimer axis. This is due to the screening exerted by the ns(2) electrons on the hole in the (n - 1)d shell. Including the spin-orbit coupling perturbatively in the DIM energy matrix has a major effect on the lowest potential energy surface of the (2)P manifold, the one for Cu allowing the formation of a "belt" of five He atoms while the one for Au being completely repulsive. Conversely, spin-orbit coupling has only a weak effect on the (2)D manifold due to the nearly degenerate nature of the diatomic potentials. Structural and energetic results from DMC have been used to support experimental indications for the formation of metastable exciplexes or the opening of non-radiative depopulation channels in bulk and cold gaseous He.Physical Chemistry Chemical Physics 09/2013; · 4.20 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Interactions between bare and tri-solvated (Eigen) hydronium with water and methanol is investigated with ab initio methods. Unexplored configurations on the oxygen side of H3O+ are covered to improve our understanding of the solvation energetics. Results indicate that the interaction between the charged and polar species, with oxygen atoms oriented toward H3O+ oxygen (‘back to back’ or ‘O-side’ to ‘O-side’ interaction), is similar to or stronger than a hydrogen bond. The repulsion along the O–O directrix onset in the range 2.2–2.7 Å, indicating H2O short-range O–O contacts with (H3O+) as possible despite water coordination.Chemical Physics Letters 01/2013; 555:51–56. · 2.15 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Mixed ammonia–hydrogen molecule clusters [H 2 –(NH 3) n ] have been studied with the aim of explor-ing the quantitative importance of the H 2 quantum motion in defining their structure and energetics. Minimum energy structures have been obtained employing genetic algorithm-based optimization methods in conjunction with accurate pair potentials for NH 3 –NH 3 and H 2 –NH 3 . These include both a full 5D potential and a spherically averaged reduced surface mimicking the presence of a para-H 2 . All the putative global minima for n ≥ 7 are characterized by H 2 being adsorbed onto a rhomboidal ammonia tetramer motif formed by two double donor and two double acceptor ammonia molecules. In a few cases, the choice of specific rhombus seems to be directed by the vicinity of an ammo-nia ad-molecule. Diffusion Monte Carlo simulations on a subset of the species obtained highlighted important quantum effects in defining the H 2 surface distribution, often resulting in populating rhom-boidal sites different from the global minimum one, and showing a compelling correlation between local geometrical features and the relative stability of surface H 2 . Clathrate-like species have also been studied and suggested to be metastable over a broad range of conditions if formed.The Journal of Chemical Physics 01/2013; 139(139). · 3.12 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**The effect of quantum mechanics (QM) on the details of the nucleation process is explored employing Ne clusters as test cases due to their semi-quantal nature. In particular, we investigate the impact of quantum mechanics on both condensation and dissociation rates in the framework of the microcanonical ensemble. Using both classical trajectories and two semi-quantal approaches (zero point averaged dynamics, ZPAD, and Gaussian-based time dependent Hartree, G-TDH) to model cluster and collision dynamics, we simulate the dissociation and monomer capture for Ne(8) as a function of the cluster internal energy, impact parameter and collision speed. The results for the capture probability P(s)(b) as a function of the impact parameter suggest that classical trajectories always underestimate capture probabilities with respect to ZPAD, albeit at most by 15%-20% in the cases we studied. They also do so in some important situations when using G-TDH. More interestingly, dissociation rates k(diss) are grossly overestimated by classical mechanics, at least by one order of magnitude. We interpret both behaviours as mainly due to the reduced amount of kinetic energy available to a quantum cluster for a chosen total internal energy. We also find that the decrease in monomer dissociation energy due to zero point energy effects plays a key role in defining dissociation rates. In fact, semi-quantal and classical results for k(diss) seem to follow a common "corresponding states" behaviour when the proper definition of internal and dissociation energies are used in a transition state model estimation of the evaporation rate constants.The Journal of Chemical Physics 07/2012; 137(1):014304. · 3.12 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**We construct a set of analytical potential energy models for the interaction of the para-H-2-ammonia, ortho-H-2-ammonia, and classical-H-2-ammonia dimers by fitting a function to a number of ab initio energies. The minimum energy for the classical-H-2-ammonia dimer is at -1.1164 mhartree. The classical-H-2 molecule is the 'hydrogen bond donor' on the side of the lone pair on the nitrogen atom. The minimum energy obtained for the para-H-2-ammonia is -0.289 mhartree, and for ortho-H-2-ammonia is -0.281 mhartree. The ammonia molecule is the hydrogen bond donor in both dimers. (C) 2012 Elsevier B. V. All rights reserved.Chemical Physics Letters 05/2012; 535:49-55. · 2.15 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**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.Biomacromolecules 02/2012; 13(3):833-41. · 5.37 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**The mechanism responsible for the production of branched polyethylene from ethylene feed during its polymerization promoted by dichloro{1,4-dithiabutanediyl-2,2'-bis(4,6-di-tert-butyl-phenoxy)} titanium complex 1 activated by methylalumoxane (MAO) was investigated by using a density functional theory (DFT) approach. The following processes, chain propagation (CP) reaction, beta hydrogen elimination (beta HE, hydrogen elimination from an alkyl chain to the metal in the absence of a coordinated alkene) and transfer (beta HT, hydrogen transfer from the alkyl chain to a coordinated alkene) were considered to individuate the possible competing reactions responsible for the production of ethylene oligomers. The latter two processes are generally ascribed as termination reactions for the chain propagation. We found that beta HT is the more plausible termination pathway leading to the formation of oligomers that can be reinserted in the growing chain, and thus giving the branched polyethylene. Furthermore, by comparing the energetic profile of CP and beta HT for the analogue zirconium compound 2 we found that beta HT is 50 times more competitive with CP than for the Ti-based compound. As a matter of fact, the MAO activated zirconium complex exclusively produces oligomers with even number of carbon atoms, thus confirming fairly well the calculations that indicated the possibility of tuning the catalytic activity and selectivity of this class of compounds simply by changing the nature of the metal center.Acs Catalysis. 10/2011; 1(11):1460-1468. - [Show abstract] [Hide abstract]

**ABSTRACT:**Exploiting the theoretical treatment of particles diffusing on corrugated surfaces and the isomorphism between the "particle on a sphere" and a linear molecule rotation, a new diffusion kernel is introduced to increase the order of diffusion Monte Carlo (DMC) simulations involving linear rotors. Tests carried out on model systems indicate the superior performances of the new rotational diffusion kernel with respect to the simpler alternatives previously employed. In particular, it is evidenced a second order convergence toward exact results with respect to the time step of dynamical correlation functions, a fact that guarantees an identical order for the diffusion part of the DMC projector. The algorithmic advantages afforded by the latter are discussed, especially with respect to the "a posteriori" and "on the fly" extrapolation schemes. As a first application to the new algorithm, the structure and energetics of O(2)@He(n) (n = 1-40) clusters have been studied. This was done to investigate the possible cause of the quenching of the reaction between O(2) and Mg witnessed upon increasing the size of superfluid He droplets used as a solvent. With the simulations on O(2) indicating a strong localization in the cluster core, the behaviour as a function of n is ascribed to the extremely fluxional comportment of Mg@He(n), which dwells far from the droplet center, albeit being solvated, when n is large.The Journal of Chemical Physics 09/2011; 135(11):114504. · 3.12 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**We report a theoretical investigation of the solution properties of Cu and Ag atoms dissolved in He clusters. Employing our recent ab initio ground state pair potential for Me-He (Me = Ag, Cu), we simulated the species Me@He (n) (n = 2-100) by means of diffusion Monte Carlo (DMC) obtaining exact information on their energetics and the structural properties. In particular, we investigated the sensitivity of structural details on the well depth of the two interaction potentials. Whereas Ag structures the first He solvation layer similarly, to some extent, to a positive ion such as Na(+), Cu appears to require the onset of a second solvation shell for a similar dense structure to be formed despite an interaction well of 28.4 μhartree. An additional signature of the different solution behavior between Ag and Cu appears also in the dependence of the energy required to evaporate a single He atom on the size of the MeHe(n) clusters. The absorption spectrum for the (2)P ← (2)S excitation of the metals was also simulated employing the semi-classical Lax approximation to further characterize Me@He(n) (n = 2-100) using novel accurate interaction potentials between He and the lowest (2)P state of Ag and Cu in conjunction with the Diatomic-in-Molecules approach. The results indicated that Ag exciplexes should not form via a direct vertical excitation into an attractive region of the excited manifolds and that there is an interesting dependence of the shape of the Cu excitation bands on the local structure of the first solvation shell.The Journal of Physical Chemistry A 03/2011; 115(25):7141-52. · 2.77 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**The relative rotamer, dimer and tautomer concentrations of diacetamide have been studied by means of infrared spectroscopy, with the recorded spectra being analyzed employing results from density functional theory calculations. It is observed that the cis-trans monomeric form of diacetamide (1) is found to be the most stable isomer in all studied solvents, with trans-trans diacetamide (2) being found to be 20% of total diacetamide in methanol. While the dimer form of diacetamide (3) is present only in carbon-tetrachloride (about 34% of the total), its tautomeric forms (4, 5) are not favorable in any of the studied solvents. (C) 2011 Elsevier B.V. All rights reserved.Vibrational Spectroscopy 01/2011; 57(2):294-299. · 1.75 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Quantum Monte Carlo (QMC) methods sample the wave function, in principle the exact one, instead of optimizing analytical functions as standard ab initio approaches. They have emerged as suitable alternative to ab initio methods to deal with the dynamical correlation in the description of the electronic structure of atoms and molecules. Differently from standard quantum chemistry approaches, QMC enjoys several features that make it the method of choice for complicated systems. Among these features, it is important to notice its intrinsically parallelizable nature, the slow (∼N 3 ) scaling of the computational cost with number of particles N, the limited amount of memory required and its ability to deal with substantially different systems within the same theoretical/algorithmic structure (e.g. it can be easily applied to both bosons and fermions, to the description of systems containing electrons and positrons as well as of vibrational properties of molecules). One of the strongest points characterizing QMC is the fact that it may use any kind of basis sets, albeit uncommon ones, depending on the species. This fact allows QMC either to quickly converge to the exact answer in the case of bosonic systems, or to easily recover 90–95% of the correlation energy in electronic species without the inverse cubic convergence with respect to the size of the basis set that plagues more common ab initio methods. Thanks to these characteristics, different flavours of QMC have been applied to a wide set of species/problems spanning a range that stretches from molecules as small as water up to pieces of bulk matter as large as silicon and germanium crystals and that includes molecules such as porphyrins and C 20 . Despite the strong points highlighted, QMC suffers from a relatively high computational cost mainly due to the necessity of evaluating many times a reference wave function. Thus, a substantial reduction of this cost may come from central processing units (CPU) particularly tailored to compute exponentials, polynomials and rational functions, from better algorithms that require less function evaluations and from variance reduction techniques. Besides, more robust approaches to reduce the so called “nodal error” would help improve the already appreciable accuracy afforded by QMC. With the above issues improved, there are several avenues that would become possible to pursue on a routine basis. Among these, we foresee the calculations of intermolecular interactions for very large systems (e.g. parts of DNA with or without interacting species), the calculation of nuclear magnetic resonance (NMR) parameters for difficult systems, the automatic optimization of molecular structures and, even better, the chance of running molecular dynamics simulation à la Car-Parrinello using QMC computed atomic forces. In these circumstances, the study of phase transitions, bulk matter, interfaces and large biological systems may reach an unexpected level of accuracy that is currently unavailable due to methodological limitations.Solving the Schrodinger equation. Has everithing been tried?, Edited by Paul Popelier, 01/2011: pages 237269; Imperial College Press., ISBN: 1-84816-724-5 -
##### Article: Sequential growth simulation of (NH3)(n) clusters (n=2-8) in ultracold superfluid environment

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**ABSTRACT:**The growth of ammonia cluster (NH3)(n) (n = 2-8) in He droplets was studied using a polarizable potential assuming ultrafast heat dissipation and preventing the rearrangement of cluster structures. Energies, structures and isomer statistical distributions were obtained showing that a few high energy isomers may form with a probability similar to low energy ones and that ammonia clusters behave differently from (H2O)(n) for n >= 5. Guide by our theoretical analysis, we suggest the need for re-interpreting recent experimental IR spectra of (NH3)(n) in He droplets for 5 <= n <= 8. (C) 2011 Elsevier B.V. All rights reserved.Chemical Physics Letters 01/2011; 514(1-3):16-20. · 2.15 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Variational Monte Carlo, diffusion Monte Carlo, and stereographic projection path integral simulations are performed on eight selected species from the (NH(3))(n), (ND(3))(n), (NH(2)D)(n), and (NH(3))(n-1)(ND(3)) clusters. Each monomer is treated as a rigid body with the rotation spaces mapped by the stereographic projection coordinates. We compare the energy obtained from path integral simulations at several low temperatures with those obtained by diffusion Monte Carlo, for two dimers, and we find that at 4 K, the fully deuterated dimer energy is in excellent agreement with the ground state energy of the same. The ground state wavefunction for the (NH(3))(2-5) clusters is predominantly localized in the global minimum of the potential energy. In all simulations of mixed isotopic substitutions, we find that the heavier isotope is almost exclusively the participant in the hydrogen bond.The Journal of Chemical Physics 12/2010; 133(21):214301. · 3.12 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Chemical and physical properties of polymeric species in solution strongly depend on their structure, which can be modulated by covalently linking substituents of different solubility. In this work, the effect of changing the interaction strength and fractional loading of hydrophobic substituents on semi-flexible hydrophilic polymers of varying chain length is studied by means of Monte Carlo simulations and coarse grained model potentials. The latter are chosen in order to provide a more factual representation of a chain in diluted solution, introducing substituent flexibility and realistic torsional and bending potentials. Upon increasing the number and the interaction strength of the substituents, our results indicate a less steep rise of the chain gyration radius and "end to end" distance for the chain length than predicted for an unsubstituted polymer in an almost good solvent. Moreover, a "disordered to compact" structural transition appears. In parallel, the formation of hydrophobic nuclei and the consequent appearance of flexible polymer loops grafted to the semi-rigid cores is witnessed. The core formation resembles a nucleation phenomenon, where the change in the interaction between the substituents modulates the free energy surface for the aggregation process similarly to the change in chemical potential. Interestingly, it has been found that a single chain containing a sufficiently high number of interacting substituents may give rise to the formation of multiple cores, suggesting that the chain stiffness may play a role in defining the structure of the free energy minimum. (C) 2010 Elsevier Ltd. All rights reserved.Polymer. 01/2010; 51(15):3582-3589. - [Show abstract] [Hide abstract]

**ABSTRACT:**Computational techniques (second order Møller-Plesset MP2 perturbation theory in conjunction with medium and large size basis sets) are applied to explore structural aspects of a hydrogen-bonded tetrameric cluster of methanol molecules, based geometrically on a tetrahedral arrangement of the four oxygen atoms of the cluster. The hydrogen-bonded structures that represent minima on the potential energy surface are established, and the complete set of pathways that allow interconversion between these structures through "switching" of the hydrogen bonding arrangement are elucidated. The implications of these results in terms of dynamic properties of the cluster are discussed.Physical Chemistry Chemical Physics 12/2009; 11(47):11340-6. · 4.20 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**In this work, collisions between rotating atomic clusters composed of Lennard-Jones (LJ(n)) particles and an identical projectile have been investigated by means of trajectory simulations as a function of the cluster angular momentum J and internal energy E, and for different values of the projectile impact parameter b and relative velocity v(p). As expected, the collision (P(c)(b)) and capture [or sticking P(s)(b)] probabilities are found to decay below unity for values of b larger than the average surface radius of the cluster, with dP/db being strongly dependent on v(p). Both P(c)(b) and P(s)(b), however, appear to be largely insensitive to the modulus of the cluster angular momentum |J| and only weakly dependent on E for collisions involving target clusters with a lifetime tau>100 ps. The latter findings are interpreted as indicating the absence of strong changes in the structure of the target as a function of |J| and E. The comparison between the dissociation lifetime (tau(dyn)) of the postcapture complexes (LJ(n+1)(*)) obtained continuing trajectories after monomer capture and the one computed from the fragmentation of statistically prepared clusters (tau(stat)) supports the validity of a two-step capture-dissociation model; similarly, the comparison between the average amount of energy exchanged during trajectories (DeltaE(dyn)) in the process LJ(n)+LJ-->LJ(n+1)(*)-->LJ(n)+LJ and the one predicted by statistical simulations (DeltaE(stat)) suggests a fast statistical energy redistribution in the collisional complex even for very short tau(dyn) (e.g., 40 ps). In the case of projectiles aimed at the edge of the cluster [(grazing collisions, P(c)(b)<1]; however, the time elapsed between formal collision and dissociation, tau(coll), is such that tau(coll)<tau(stat) and the trajectories indicate the presence of ballistic dynamics and of a weak energy exchange (DeltaE(coll)<DeltaE(dyn), with DeltaE(coll) being the average energy exchanged during collisions). The relevance of these results to the study of gas phase nucleation and to the possibility of building a fully microcanonical framework for its description is discussed.The Journal of Chemical Physics 09/2009; 131(12):124309. · 3.12 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Classical and quantum simulations of ammonia clusters in the dimer through the hendecamer range are performed using the stereographic projection path integral. Employing the most recent polarizable potential to describe intermolecular interactions, energetic and structural data obtained with our simulations provide support for a more fluxional or flexible nature at low temperature of the ammonia dimer, pentamer, and hexamer than in the other investigated species. The octamer and the hendecamer display a relatively strong melting peak in the classical heat capacity and a less intense but significant melting peak in the quantum heat capacity. The latter are shifted to lower temperature (roughly 15 and 40 K lower, respectively) by the quantum effects. The features present in both classical and quantum constant volume heat capacity are interpreted as an indication of melting even in the octamer case, where a large energy gap is present between its global minimum and second most stable species. We develop a first order finite difference algorithm to integrate the geodesic equations in the inertia ellipsoid generated by n rigid nonlinear bodies mapped with stereographic projections. We use the technique to optimize configurations and to explore the potential surface of the hendecamer.The Journal of Chemical Physics 08/2009; 131(3):034312. · 3.12 Impact Factor -
##### Article: On possible simplifications in the theoretical description of gas phase atomic cluster dissociation.

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**ABSTRACT:**In this work, we investigate the possibility of describing gas phase atomic cluster dissociation by means of variational transition state theory (vTST) in the microcanonical ensemble. A particular emphasis is placed on benchmarking the accuracy of vTST in predicting the dissociation rate and kinetic energy release of a fragmentation event as a function of the cluster size and internal energy. The results for three Lennard-Jones clusters (LJ(n), n=8,14,19) indicate that variational transition state theory is capable of providing results of accuracy comparable to molecular dynamics simulations at a reduced computational cost. Possible simplifications of the master equation formalism used to model a dissociation cascade are also suggested starting from molecular dynamics results. In particular, it is found that the dissociation rate is only weakly dependent on the cluster total angular momentum J for the three cluster sizes considered. This would allow one to partially neglect the J-dependency of the kinetic coefficients, leading to a substantial decrease in the computational effort needed for the complete description of the cascade process. The impact of this investigation on the modeling of the nucleation process is discussed.The Journal of Chemical Physics 03/2009; 130(8):084108. · 3.12 Impact Factor

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