Dynamics Simulation of Water at Critical and Supercritical Conditions

Molecular dynamics simulation of water at critical and supercritical conditions 

A preliminary molecular dynamics simulation was performed. To this end, SCF-MI and MO-VB calculations were carried out on water dimers and trimers and a new parameterisation of a NCC-like (Niesar-Corongiu-Clementi) [53] potential was accomplished [10d]. The 7s4p2d/4s2p small basis set proposed by Millot and Stone [40] was employed. The Hartree Fock limit results obtained in our previous work were reproduced using this compact basis set also equilibrium geometry and binding energy values compared well with the experimental results. 

By fitting the MO-VB ab initio results, the parameters of a new NCC-like potential have been determined. The ab initio MO-VB calculations include the determination of the interaction energy relative to 225 configurations of the 

The molecular dynamics simulation was performed using the MOTECC suite of programs [54] in the context of a microcanonical statistical ensemble. The system considered is a cube, with periodic boundary conditions, which contains 343 water molecules. The molecular dynamic simulation of water performed at ambient conditions revealed good agreement with experimental measurements. The main contribution to the total potential energy comes from the two-body term, while the many-body polarisation term contribution amounts to 23% of the total potential energy. Some of the properties calculated during the simulation are reported in Table 3. 

The number of first neighbours around a central water molecule, calculated by integrating the correlation function goo(r) up to the position of the first minimum, was 5.5, consistent with a local tetrahedron (with addition of contributions from some interstitial coordination). 

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Famulari, A., Specchio, R., Sironi, M., and Raimondi, M. (1998) New basis set superposition error free ab initio MO-VB interaction potential molecular dynamics simulation of water at critical and supercritical conditions, J. Chem. Phys., 108, 3296-3303. 

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