Diffusive and quantum effects of water properties in different states of matter

Abstract

The enthalpy, entropy, and free energy of water are important physical quantities for understanding many interesting phenomena in biological systems. However, conventional approaches require different treatments to incorporate quantum and diffusive effects of water in different states of matter. In this work, we demonstrate the use of the two-phase thermodynamic (2PT) model as a unified approach to obtain the properties of water over the whole phase region of water from short (∼20 ps) classical molecular dynamics trajectories. The 2PT model provides an effective way to separate the diffusive modes (gas-like component) from the harmonic vibrational modes (solid-like component) in the vibrational density of states (DoS). Therefore, both diffusive and quantum effect can be properly accounted for water by applying suitable statistical mechanical weighting functions to the DoS components. We applied the 2PT model to systematically examine the enthalpy, entropy, and their temperature dependence of five commonly used rigid water models. The 2PT results are found to be consistent with those obtained from more sophisticated calculations. While the thermodynamic properties determined from different water models are largely similar, the phase boundary determined from the equality of free energy is very sensitive to the small inaccuracy in the values of enthalpy and absolute entropy. The enthalpy, entropy, and diffusivity of water are strongly interrelated, which challenge further improvement of rigid water model via parameter fitting. Our results show that the 2PT is an efficient method for studying the properties of water under various chemical and biological environments.

Entropy of flexible liquids from hierarchical force–torque covariance and coordination

“…31,32 It has been shown 33 that the matrix of Fourier transforms of the velocity autocorrelation function is equivalent to the force covariance matrix 29,30 as well as the ensemble average of the Hessian. A notable advantage of harmonic approaches is that quantum effects can be incorporated easily, [34][35][36][37][38] especially important for covalent bonds because of their high frequency.…”

Two Faces of the Two Phase Thermodynamic Model

“…The 2PT abbreviation refers to two conceptually different calculation procedures of the heat capacity in different articles. The classical heat capacities were also taken into account in the calculation of the 2PT heat capacity of water, 9,10,17,33 but in the case of organic solvents classical heat capacities were discarded. [19][20][21] According to refs 19-21 we refer to 1PT and 2PT heat capacities that do not contain anharmonic corrections calculated from classical values.…”

“…This definition of 2PT+AC heat capacity may correspond to previous 2PT calculations in the literature where the classical heat capacities were also taken into account. 9,10,17,22,33 3 Methods…”

Two Faces of the Two Phase Thermodynamic Model

“…2PT and 1PT+AC methods were successfully applied for the calculation of thermodynamic properties of several systems such as Lennard-Jones fluids, 3,5 water, 2,4,[6][7][8][9][10][11][12][13][14][15] aqueous solutions, 16,17 molten salts, 18 organic liquids, [19][20][21] carbon dioxide, 22 urea, 23 ionic liquids, [24][25][26][27] carbohydrates, 28 cellulose, 29 mixtures 30 and interfaces. [31][32][33][34][35][36] Lately, 2PT was used for the definition of the Frenkel line.…”

“…The 2PT abbreviation refers to two conceptually different calculation procedures of the heat capacity in dif-ferent articles. The classical heat capacities were also taken into account in the calculation of the 2PT heat capacity of water, 9,10,17,33 but in the case of organic solvents classical heat capacities were discarded. [19][20][21] According to refs 19-21 we refer to 1PT and 2PT heat capacities that do not contain anharmonic corrections calculated from classical values.…”