# How do you incorporate hydrogen bonding into molecular simulations?

How do computer simulations of liquid hydrogen-bonding systems incorporate hydrogen bonding? Since I think hydrogen bonding has mixed covalent and electrostatic character, wouldn't you need something beyond a classical force field? Yet, for example, simulations of water using a simple classical model like spc/e reproduce the structure, including the tetrahedral coordination.

• People have tried adding special hydrogen bonding contributions to FF's. They don't catch on, possibly because hydrogen bonding is over-rated. Hydrogens are elements with electrons, just like the other elements. You can parameter fit them just the same as the others and unsurprisingly, get as reasonable of results as the others. Hydrogen bonding makes for a good catch phrase though. Oct 28 at 15:05

Covalent character does not necessarily mean you can't represent a interaction by a classical force field. With a sufficiently complex force field, you can represent a lot of chemistry (for example, ReaxFF). I should also mention that ReaxFF has a special term for hydrogen bonds: $$E_\mathrm{Hbond} = p_\mathrm{hb1} \left[ 1 - \exp(p_\mathrm{hb2} \mathsf{BO}_\mathrm{XH}) \right] \exp \left[ p_\mathrm{hb3} \left( \frac{r^\circ_\mathrm{hb}}{r_\mathrm{HZ}} + \frac{r_\mathrm{HZ}}{r^\circ_\mathrm{hb}} - 2 \right) \right] \sin^8 \left( \frac{\Theta_\mathrm{XHZ}}{2} \right),$$ where $$\mathsf{BO}_\mathrm{XH}$$ is the bond order between the donor atom and the hydrogen atom (calculated as detailed in the supporting information of the paper), $$r_\mathrm{HZ}$$ is the distance between hydrogen atom and the acceptor atom, $$\Theta_\mathrm{XHZ}$$ is the donor-hydrogen-acceptor angle, and all other variables are free parameters defined in the force field.