A flexible model for water based on TIP4P/2005

A new flexible water model, TIP4P/2005f, is developed. The idea was to add intramolecular degrees of freedom to the successful rigid model TIP4P/2005 in order to try to improve the predictions for some properties, and to enable the calculation of new ones. The new model incorporates flexibility by means of a Morse potential for the bond stretching and a harmonic term for the angle bending. The parameters have been fitted to account for the peaks of the infrared spectrum of liquid water and to produce an averaged geometry close to that of TIP4P/2005. As for the intermolecular interactions, only a small change in the σ parameter of the Lennard-Jones potential has been introduced. The overall predictions are very close to those of TIP4P/2005. This ensures that the new model may be used with the same confidence as its predecessor in studies where a flexible model is advisable.