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

3In 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 H-2 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 energy calculation. The same approach could be exploited also in computing the derivative of other energy-dependent quantum-mechanical observables. (C) 2003 American Institute of Physics.openM. Casalegno;M. Mella;A. M. RappeM., Casalegno; Mella, Massimo; A. M., Rapp

Molecular dynamics simulations of the solvent- and thermal history-dependent structure of the PCBM fullerene derivative

The fullerene derivative PCBM ([6,6]phenyl-C61-butyric acid methyl ester) is one of the best electron acceptors used so far in solution-processed organic photovoltaic devices. The reasons for this success depend partly on its favourable electronic properties, partly on its solubility in common organic solvents and plausibly also on the possibility to optimize its structure and morphology by postdeposition treatments (solvent or thermal annealing). The latter feature is still largely a matter of speculation, as experimentally validated structural models of PCBM molecular organization within the devices are still unavailable. This structural characterization is non-trivial, given that poorly ordered PCBM nanocrystals and amorphous domains appear to often coexist in bulk-heterojunction films based on this system. Here we address some of these issues using molecular dynamics (MD) simulations. Our starting points are the only two published PCBM crystal structures, which were obtained by crystallization from oDCB (ortho-dichlorobenzene) and MCB (monochlorobenzene). Both contain guest molecules of the specific solvent. We simulated their thermal behavior, from room temperature up to their apparent melting points. Additional MD simulations involved model crystals obtained by removing solvent molecules from these co-crystal structures. Models that can apply to the amorphous phase or to nanocrystalline samples have been obtained by cooling molten PCBM, after removing the solvent at different stages in the simulation. Their densities are close to the experimental values and they present a well interconnected network of fullerene moieties, where each of them has an average of seven close neighbours available for charge hopping. Pre- and post-melting structural features such as intermolecular pair distribution functions are discussed in the framework of organic solar cell production and host–guest system dynamics

Hyaluronic acid-based hydrogels: Drug diffusion investigated by HR-MAS NMR and release kinetics

Hydrogels based on hyaluronic acid (HA) and agarose-carbomer (AC) raised an increasing interest as drug delivery systems. The complex architecture of the polymer network, such as mesh size, HA molecular weight and drug-polymer non covalent interactions across the 3D polymer matrix strongly influence the release capability/profile of these materials. In this study, AC-HA hydrogels with different mesh sizes have been prepared and characterised. High Resolution Magic Angle Spinning (HR-MAS) NMR spectroscopy has been used to investigate the motion of two drugs, such as ethosuximide (neutral molecule) and sodium salicylate (net negative charge) within the AC and AC-HA hydrogel networks. Analysis of the experimental data provides evidence of superdiffusive motion for all formulations containing sodium salicylate, while ethosuximide molecules undergo unrestricted diffusion within the gel matrix. We further speculate that the superdiffusive motion, observed at the nanoscale, can be responsible for the faster release of sodium salicylate from all hydrogel formulations