1,721,010 research outputs found
Stability of (He2HeM)-He-3-He-4 and (He3HeM)-He-3-He-4 L=0, 1 clusters
No full textWe have studied the stability of mixed 3He/4He clusters in L 1⁄4 0
and L 1⁄4 1 states by the diffusion Monte Carlo method, employing the Tang-Toennies-Yiu (TTY) He-He potential. The clusters 3He4HeM (L 1⁄4 0, S 1⁄4 12) and 3He2 4HeM (L 1⁄4 0, S 1⁄4 0) are stable for M >1, while to bind two 3He in a triplet state the minimum number of 4He is four. Considering clusters with three 3He, 3He3 4He4 is the smallest stable system in the L 1⁄4 1 state, while 3He3 4He8 is the smallest stable system in the L 1⁄4 0 state
A compact boundary-condition- determined wavefunction for two-electron atomic systems
No full textHighly compact wavefunctions with a clear physical meaning for the He atom and He-like isoelectronic ions for Z = 1–10 are written as a symmetrized product of exp[(ar + br2)/(1 + r)] electron–nucleus terms and an electron–electron Jastrow factor to satisfy the correct asymptotic behaviour both at short and long interparticle distances. Some parameters are chosen to satisfy exactly the cusp conditions, while the others are optimized by variational Monte Carlo calculations. The wavefunction energy is within 2 millihartrees from the non-relativistic limit in the entire Z-range, improving previously published work on similar compact wavefunctions. We tested the validity of the 'coalescence wavefunction' approximation. The Z-dependence of the optimized parameters allows us to write a general form of the wavefunction, using Z as an explicit parameter and four parameters independent of Z. We checked the validity of this wavefunction on the case Z = 30
Stability and positron annihilation of positronium hydride L=0,1,2 states: A quantum Monte Carlo study
No full textStates of positronium hydride having different angular momenta have been studied by means of quantum Monte Carlo techniques. Explicitly correlated wave functions for different states have been obtained using the variational Monte Carlo optimization method. These wave functions have been used in variational Monte Carlo and diffusion Monte Carlo (DMC) simulations to compute energies, annihilation rates, and other observables. Our DMC results compare well with the best published variational ground-state binding energy and show that positronium hydride has metastable states with angular momentum L = 1 and 2 above the ground-state dissociation threshold. The values of the other observables for the ground state are comparable with the best variational calculations. The results for the L = 1 and 2 states are used to discuss a proposed model for the annihilation of positrons in alkali hydrides crystals
A Monte Carlo simulation of liquid 1, 2-dimethoxyethane
No full textMonte Carlo simulations have been carried out for liquid 1,2-dimethoxyethane in the NVT ensemble at 298 K with 125
and 216 molecules. The intermolecular interactions are described as sums of Lennard-Jones and Coulomb terms. The
intramolecular rotations are described by an analytical potential function fitted to MM2 energies. The heat of vaporization
is found in good agreement with the experimental value. While the gas phase is a mixture of gauche and anti conformations,
in the pure liquid the gauche conformation is preferred, as found experimentally. The liquid is disordered with high coordination
numbers and the most evident packing effect is shown by terminal CH3 groups
Stability of four-body systems in three and two dimensions: A theoretical and quantum Monte Carlo study of biexciton molecules
No full textThe stability of four-body systems (m(a)(+)m(b)(+)m(1)(-)m(2)(-)) in three (3D) and two dimensions (2D) is discussed using accurate numerical results obtained by means of diffusion Monte Carlo calculations. In 3D, we extend our proof of the stability for the class of systems (m(a)(+)m(b)(+)m(1)(-)m(1)(-)), showing that they are stable against the dissociation in (m(a)(+)m(1)(-);) and (m,fm;) for any value of the mass ratio m(a)(+)/m(b)(+). In 2D, using the ground-state energy of the dipositronium, it is possible to prove that the stability of four-body systems follows the same scenario. We also give upper and lower bounds to the binding energies for the class (M(+)M(+)m(-)m(-)) in 2D, useful to discuss the relative stability of biexciton molecules in semiconductors
Time step bias improvement in diffusion Monte Carlo simulations
No full textA Makri-Miller approximation to the exact propagator and the improved split-operator propagator proposed by Drozdov are implemented within the diffusion Monte Carlo method for the simulation of boson systems, and confronted with the Trotter formula and with the importance sampling technique. As a preliminary approach, we compute analytically the time step bias of the mean energy for the different propagators in the simple case of the harmonic oscillator. These results indicate the improved split-operator propagator as the most accurate. Simulations on one- and three-dimensional model systems confirm the analytical results showing that this propagator is very efficient in reducing the time step bias, therefore improving the efficiency of the algorithm
Quantum Monte Carlo estimators for the positron-electron annihilation rate in bound and low-energy scattering states
No full textVariational and exact estimators for the positron-electron annihilation rate in bound states of systems containing a positron in the framework of quantum Monte Carlo methods are presented. The modification needed to compute the effective number of electrons Z(eff) when scattering states are concerned is also discussed. The algorithms are tested against four cases for which close to exact results are available, finding an overall good agreement. The systems are Ps(-), PsH, and the s-wave scattering component of e(+)H and e(+)He
What Is the Shape of the Helium Trimer? A Comparison with the Neon and Argon Trimers
No full textDespite its apparent simplicity and extensive theoretical investigations, the issue of what is the shape of the helium trimer is still debated in the literature. After reviewing previous conflicting interpretations of computational studies, we introduce the angle?angle distribution function as a tool to discuss in a simple way the shape of any trimer. We compute this function along with many different geometrical distributions using variational and diffusion Monte Carlo methods. We compare them with the corresponding ones for the neon and argon trimers. Our analysis shows that while Ne3 and Ar3 fluctuate around an equilibrium structure that is an equilateral triangle, 4He3 shows an extremely broad angle?angle distribution function, and all kinds of three-atom configurations must be taken into account in its description. Classifying 4He3 as either equilateral or linear or any other particular shape, as was done in the past, is not sensible, because in this case the intuitive notion of equilibrium structure is ill defined. Our results could help the interpretation of future experiments aimed at measuring the geometrical properties of the helium trimer
Nonadiabatic wavefunctions as linear expansions of correlated exponentials. A quantum Monte Carlo application to H-2(+) and Ps(2)
No full textWe propose to expand the nonadiabatic solution of the Schrodinger equation as a linear combination of explicitly correlated exponentials. A series of trial wavefunctions has been optimized minimizing the variance of the local energy for the H-2(+) and dipositronium (Ps(2)) molecules in their ground state, without resorting to the Born-Oppenheimer approximation: the calculations have been performed using the variational Monte Carlo method. In a diffusion Monte Carlo simulation a h-term wavefunction allowed us to compute the exact energy of the Ps(2) system -0.51601 hartree with a variance of 0.00001 hartree. (C) 1997 Elsevier Science B.V
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