Fully Quantum Description of the Zundel Ion: Combining Variational Quantum Monte Carlo with Path Integral Langevin Dynamics.

We introduce a novel approach for a fully quantum description of coupled electron-ion systems from first principles. It combines the variational quantum Monte Carlo solution of the electronic part with the path integral formalism for the quantum nuclear dynamics. On the one hand, the path integral molecular dynamics includes nuclear quantum effects by adding a set of fictitious classical particles (beads) aimed at reproducing nuclear quantum fluctuations via a harmonic kinetic term. On the other hand, variational quantum Monte Carlo can provide Born-Oppenheimer potential energy surfaces with a precision comparable to the most-advanced post-Hartree-Fock approaches, and with a favorable scaling with the system size. In order to cope with the intrinsic noise due to the stochastic nature of quantum Monte Carlo methods, we generalize the path integral molecular dynamics using a Langevin thermostat correlated according to the covariance matrix of quantum Monte Carlo nuclear forces. The variational parameters of the quantum Monte Carlo wave function are evolved during the nuclear dynamics, such that the Born-Oppenheimer potential energy surface is unbiased. Statistical errors on the wave function parameters are reduced by resorting to bead grouping average, which we show to be accurate and well-controlled. Our general algorithm relies on a Trotter breakup between the dynamics driven by ionic forces and the one set by the harmonic interbead couplings. The latter is exactly integrated, even in the presence of the Langevin thermostat, thanks to the mapping onto an Ornstein-Uhlenbeck process. This framework turns out to be also very efficient in the case of noiseless (deterministic) ionic forces. The new implementation is validated on the Zundel ion (H5O2+) by direct comparison with standard path integral Langevin dynamics calculations made with a coupled cluster potential energy surface. Nuclear quantum effects are confirmed to be dominant over thermal effects well beyond room temperature, giving the excess proton an increased mobility by quantum tunneling.

Quantum Monte Carlo Study of the Protonated Water Dimer.

We report an extensive theoretical study of the protonated water dimer H5O2(+) (Zundel ion) by means of the highly correlated variational Monte Carlo and lattice regularized Monte Carlo approaches. This system represents the simplest model for proton transfer (PT), and a correct description of its properties is essential in order to understand the PT mechanism in more complex aqueous systems. Our Jastrow correlated AGP wave function ensures an accurate treatment of electron correlation. By exploiting the advantage of contracting the primitive basis set over atomic hybrid orbitals, we are able to limit dramatically the number of variational parameters with a systematic control on the numerical precision, a crucial ingredient in order to simulate larger systems. For both energetics and geometrical properties, our QMC results are found to be in excellent agreement with state-of-the-art coupled cluster CCSD(T) techniques. A comparison with density functional theory in the PBE approximation points to the crucial role of electron correlation for a correct description of the PT in the dimer. We prove that the QMC framework used in this work is able to resolve the tiny energy differences (∼0.3 kcal/mol) and structural variations involved in proton transfer reactions. Our approach combines these features and a favorable N(4) scaling with the number of particles which paves the way to the simulation of more realistic PT models. A test calculation on a larger protonated water cluster is carried out. The QMC approach used here represents a promising candidate to provide the first high-level ab initio description of PT in water.

Assessment of liver stiffness in subjects affected by familial combined hyperlipidaemia with hepatic steatosis.

BACKGROUND: Nonalcoholic fatty liver disease (NAFLD) represents a spectrum of diseases ranging from simple steatosis to nonalcoholic steatohepatitis (NASH) and is associated with familial combined hyperlipidaemia (FCHL). Currently, the invasive liver biopsy is considered as the gold standard for evaluating liver fibrosis (LF); however, liver stiffness measurement (LSM) by transient elastography (TE) trough FibroScan device may be employed to estimate LF noninvasively. The aim of this study was to evaluate the prevalence of NAFLD in FCHL subjects and to analyse LSM with TE to better identify those individuals with a potential risk of liver disease progression. MATERIALS AND METHODS: Sixty subjects with FCHL (38 men, 22 women, mean age 46.4 +/- 10.9 years) were included in the study. We studied biochemical parameters including lipid profile, glucose, transaminase and insulin; blood pressure and waist circumference (WC) were measured; BMI and HOMA-index were calculated. Ultrasonography was performed to assess liver steatosis and carotid intima-media thickness (IMT). Liver fibrosis was measured by FibroScan. RESULTS: Patients were classified according to have no (group 0: 19%), mild (group 1: 32%) or moderate-severe (group 2: 49%) steatosis. No difference was found between group 0 and 1 concerning all study parameters. WC (P < 0.05), BMI (P < 0.05), glucose (P < 0.05), insulin (P < 0.001), HOMA-index (P < 0.001) and LSM (6.03 +/- 1.9 Kpa vs. 4.2 +/- 0.5 Kpa, P < 0.001) were significantly higher in group 2 than groups 1 and 0. Furthermore, LSM correlated with insulin (P < 0.05), glucose (P < 0.05), HOMA-index (P < 0.001), transaminase (P < 0.01) and liver steatosis (P < 0.001). Regression analysis showed that LSM (P < 0.001) and NAFLD (P < 0.01) is associated with HOMA-index; NAFLD is also associated with WC (P < 0.05). CONCLUSION: Our results suggest that in FCHL subjects, HOMA-index, an insulin resistance index, is strongly associated with liver steatosis and its progression. Furthermore, in these subjects, we propose the transient elastography to identify and follow up patients for the progression of hepatic disease.