Adding flexible bonds support to TIP3P
Hi, this week I needed to run TIP3P calculations and noticed that the TIP3P calculator does not handle bonds, only the non-bonded interactions (Lennard-Jones and electrocstatic) for TIP3P water. This means TIP3P can currently only be used in combination with constraints (as correctly pointed out in the ASE wiki), which I think is a bit restrictive in my opinion.
I edited the calculator to add harmonic bonds (bond stretching and bond-angle bending) based on PBE parametrization for the water monomer, which I did myself with GPAW a while back (a fairly trivial calculation involving a quadratic fit to the PES of the monomer as bonds are deformed about equilibrium).
The feature can be used rather easily by passing a flexible = True
keyword to the calculator, which is False
by default to ensure backward compatibility. I wrote a test to check analytical forces and finite difference forces match. I was wondering: would it be interesting to add this to the master branch or not?
Edited file: