Computer simulation of liquids
Computer simulation of liquids
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A mean-field method is presented describing the electrostatic environment experienced by water molecule in liquid state, which is used to extract the corresponding hyper- and high-order polarizabilities. Within this approach, MD computer simulations of liquid water samples for two standard water potentials at several different temperatures are performed to characterize the distributions (specifically average values) of local fields and field gradients. The electric response properties (including non-linear contributions up to fourth-order) are then calculated using ab initio techniques in conjunction with a charge perturbation variant of a finite field method. Sets of fixed charges are used to generate the desired electric fields and electric field gradients. Calculations of dipole polarizability, hyper- and principal components of high-order polarizabilities of the water molecule in gas and liquid phase conditions are carried out at MP2 and MP4 levels of theory; the values obtained for three different liquid phase models are compared with those for gas phase. For a liquid phase water molecule the first hyperpolarizability (β) and first higher polarizability (A) increase markedly, actually changing sign. The second hyperpolarizability γ) also increases but much less dramatically, and components of the second high-order polarizability tensor (B) demonstrate a rearrangement of contributions. We observe that a less symmetrical gradient model gives the most accurate representation of liquid-phase conditions. The excellent agreement of our gas-phase values with experimental results and the most accurate previous theoretical predictions is evident of the quality of our higher order polarizabilities and theoretical models.