A Transferable Polarizable Force Field for Urea Crystals and Aqueous Solutions

Urea
is an important chemical with many biological and industrial
applications. In this work, we develop a first-principles polarizable
force field for urea crystals and aqueous solutions within the symmetry-adapted
perturbation theory (SAPT) protocol with the SWM4-NDP model for water.
We make three adjustments to the SAPT force field protocol: We augment
the carbonyl oxygen atom of urea with additional interaction sites
in order to address the “chelated” bent double hydrogen
bonds in urea, we reduce the polarizability of urea by a factor of
0.70 to reproduce experimental in-crystal dipole moments, and we re-fit
atomic pre-exponential parameters to correct the predicted liquid
structure. We find that the resulting force field is in good agreement
for the static and dynamic properties of aqueous urea solutions when
compared to experiment or first-principles molecular dynamics simulations.
The polarizable urea model accurately reproduces the crystal-solution
phase diagram in the temperature range of 261 to 310 K; for which,
it is superior to non-polarizable models. We expect that this force
field will be useful in the modeling of complex biomolecular systems
and enable studies of polarizability effects of solid–liquid
phase behavior of complex fluids.