I am studying the works of Alenka Luzar on the hydrogen bonding kinetics in liquid water. Luzar defines an autocorrelation function c(t) that gives the probability that an hydrogen bond is intact at time t, given it was intact at time zero. She defines the rate of relaxation to equilibrium as: $k...

In my phonon calculation utilizing DFPT in VASP for a monolayer material with space group 187, I've noticed that while most of the phonon bands are positive, two of them appear to be straight. What could be the explanation for this observation? [1]: https://i.stack.imgur.com/Q5eCS.png