The thermodynamic and ground state properties of the TIP4P water octamer

Abstract

Several stochastic simulations of the TIP4P ͓W. L. Jorgensen, J. Chandrasekhar, J. D. Madura, R. W. Impey, and M. L. Klein, J. Chem. Phys. 79, 926 ͑1983͔͒ water octamer are performed. Use is made of the stereographic projection path integral and the Green's function stereographic projection diffusion Monte Carlo techniques, recently developed in one of our groups. The importance sampling for the diffusion Monte Carlo algorithm is obtained by optimizing a simple wave function using variational Monte Carlo enhanced with parallel tempering to overcome quasiergodicity problems. The quantum heat capacity of the TIP4P octamer contains a pronounced melting peak at 160 K, about 50 K lower than the classical melting peak. The zero point energy of the TIP4P water octamer is 0.0348Ϯ 0.0002 hartree. By characterizing several large samples of configurations visited by both guided and unguided diffusion walks, we determine that both the TIP4P and the SPC ͓H. J. C. Berendsen, J. P. Postma, W. F. von Gunsteren, and J. Hermans, ͑Intermolecular Forces, Reidel, 1981͒. p. 331͔ octamer have a ground state wave functions predominantly contained within the D 2d basin of attraction. This result contrasts with the structure of the global minimum for the TIP4P potential, which is an S 4 cube. Comparisons of the thermodynamic and ground-state properties are made with the SPC octamer as well.

Nuclear quantum effects on the structure and the dynamics of [H2O]8 at low temperatures

“…(23) the simple classic diffusive picture proposed by Zwanzig 66 -where the dynamics of the molecules is cast in terms of individual harmonic oscillations interrupted by jumps over saddle points that destroy the coherence of the harmonic dynamics -the increment in the number of unstable frequencies would foreshadow the lowering of the melting transition of the octamer detected in previous quantum simulations 47,48 and (ii) a sensible widening of the frequency ranges describing the different modes: Note that the original energy gap at ω ∼ 1360 cm −1 that, in the classical case, separates the low frequency translational and rotational bands from the bending one, is bridged by a new set of RPMD-intermediate frequencies. More dramatic are the changes affecting the range of frequencies involved in stretching motions, which now spans a much wider, ∼2000 cm −1 − 5000 cm −1 frequency interval.…”

“…43 Considering their hydrogen bond (HB) connectivity pattern, both isomers present four water molecules acting as double-donor-single acceptors (DDSA) while the other four act as single-donor-double-acceptors (SDDA) and exhibit dangling hydrogens (H dng ). From a thermodynamic perspective, computer simulations have already revealed that these aggregates undergo solid-liquid phase transitions in a thermal range spanning, roughly, the 150 K-200 K interval, 31,34,[44][45][46] and that the explicit introduction of quantum nuclear fluctuations shifts the predicted melting temperature down by about 10-20 K. 47,48 Typically, quantum simulation studies for water have been carried out using potential energy surfaces borrowed from classical force fields for the bulk. As a possible improvement, Manolopoulos and co-workers 49 introduced the flexible q-TIP4P/F model, specifically tailored to avoid implicit quantum effects and be implemented in path-integral simulations of water.…”

Germanium Based Field-Effect Transistors: Challenges and Opportunities

“…More recently, it has been found that GeO 2 can serve as an effective IL , and the quality of the interface is strongly dependent on how the interfacial oxide is formed [102–104]. A wide variety of oxidation methods are being explored, including thermal oxidation [12,71,102,119–130], ozone or atomic oxygen exposure [131–136], high-pressure oxidation [103,104,137], radical oxidation [138], and plasma techniques [9,14,139]. …”

“…Some works have observed that suppressing Ge interdiffusion with the high-k material results in improved performance [14,140]; suggesting defects are created during the interdiffusion process. Recently, researchers have shown some success in improving interface quality by capping the GeO 2 /Ge interface with diffusion resistant high-k material [13,14,138]. …”

Infinite swapping in curved spaces

“…where dX = dx 1 dx 2 · · · dx N , P n is the permutation operator, and idnx(n, k) is the pointer to all the permutations in the coordinate-temperature list associated with temperature k. 24 The symbol x i is used to no-development of INS and PINS 24 is particularly timely, given our own interests in simulating molecular clusters, [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] and the difficulties we have encountered in the process. Molecular clusters seem to have a much richer set of rare-event like thermodynamic behaviors compared to an atomic cluster of equivalent size.…”