Thermodynamics and equilibrium structure of Ne38 cluster:: Quantum mechanics versus classical -: art. no. 154305

The equilibrium properties of classical Lennard-Jones (LJ(38)) versus quantum Ne-38 Lennard-Jones clusters are investigated. The quantum simulations use both the path-integral Monte Carlo (PIMC) and the recently developed variational-Gaussian wave packet Monte Carlo (VGW-MC) methods. The PIMC and the classical MC simulations are implemented in the parallel tempering framework. The classical heat capacity C-v(T) curve agrees well with that of Neirotti et al. [J. Chem. Phys. 112, 10340 (2000)], although a much larger confining sphere is used in the present work. The classical C-v(T) shows a peak at about 6 K, interpreted as a solid-liquid transition, and a shoulder at similar to 4 K, attributed to a solid-solid transition involving structures from the global octahedral (O-h) minimum and the main icosahedral (C-5v) minimum. The VGW method is used to locate and characterize the low energy states of Ne-38, which are then further refined by PIMC calculations. Unlike the classical case, the ground state of Ne-38 is a liquidlike structure. Among the several liquidlike states with energies below the two symmetric states (O-h and C-5v), the lowest two exhibit strong delocalization over basins associated with at least two classical local minima. Because the symmetric structures do not play an essential role in the thermodynamics of Ne-38, the quantum heat capacity is a featureless curve indicative of the absence of any structural transformations. Good agreement between the two methods, VGW and PIMC, is obtained. The present results are also consistent with the predictions by Calvo et al. [J. Chem. Phys. 114, 7312 (2001)] based on the quantum superposition method within the harmonic approximation. However, because of its approximate nature, the latter method leads to an incorrect assignment of the Ne-38 ground state as well as to a significant underestimation of the heat capacity. (c) 2005 American Institute of Physics.