[gmx-users] Free energy of ice crystals (Thermodynamic Integration)

[Nathan K Houtz]

Hello,


I would like to perform some free energy calculations of ice Ic (cubic ice) at various temperatures. However, I'm having difficulty finding information on how Gromacs can be used to do this. The goal is to gradually turn off VDW and Coulomb interactions, just as one would do in a liquid, but also turn on virtual spring-like potentials to create an Einstein crystal, which is where I'm having trouble. I know you can fix atom locations, and considering bond potentials are spring-like, my best idea is to create a virtual copy of each molecule (not virtual sites, but just atoms that have no charge and don't interact with other atoms/molecules), fix it in the crystal lattice, and create bonds between corresponding atoms with an equilibrium distance of 0. But this means I have to simulate twice as many atoms, and more importantly I don't know how to couple only some of a molecule's bonds with lambda. Am I even on the right track, or is there a more straightforward way to do this? Are there any tutorials for free energies of solids that I could take a look at? (I couldn't find any)


Thanks for your help!

N. H.