# What is Hydrogen bond Auto-correlation function

Can somebody explain the Hydrogen bond Auto-correlation function in Computational Chemistry (Molecular Dynamics and Simulations) and what does Intermittent and Continuous Hydrogen bond means?

Why the graph decays over time? and it's applications?

This is with reference to the paper: Chandra, A. Effects of Ion Atmosphere on Hydrogen-Bond Dynamics in Aqueous Electrolyte Solutions. Phys. Rev. Lett. 2000, 85 (4), 768–771. DOI: 10.1103/PhysRevLett.85.768.

• @D.H.N in general, you should try to edit your prior question to get it reopened rather than posting essentially the same question again. – Tyberius Jul 6 '18 at 3:31
• @Tyberius i am unable to edit mt prior question since it has been locked and hence i posted the question again. – D.H.N Jul 6 '18 at 4:06
• @D.H.N As far as I can see, your question was put on hold, which is not the same as it being locked. With it put on hold, when you edit the question, it gets sent to other users to vote on if the question has been improved enough to reopen. – Tyberius Jul 6 '18 at 4:11
• @Tyberius if i go to the edit option of my previous post which was put on hold, it shows a pop-up which states that the "Post is locked" and also it shows "This post is locked and cannot be edited". Hence i am unable to edit that. – D.H.N Jul 6 '18 at 6:21

In eqn 1 and 2 the correlation functions are written in the form $<A B >/<A>$ , the denominator is the average of $A$ and the numerator the correlation function. This is the the integral product $U(t_d)=\int A(t) B(t-t_d)dt$ where $t_d$ is some time delay. When $A$ and $B$ are the same thing then this is the auto-correlation. It measure how similar the signal A is to B at different times. If $A$ and $B$ are random signals (such as white noise, or from a random number generator) then $U$ is 1 at zero time delay (both signals exactly superimposed) and zero at all other delays and this is because the correlation has no 'memory'; any point in a random signal is not related to any other. If $A$ and $B$ are constant then $U$ is one at all times, i.e perfect 'memory'. In your example the H bonds have a certain lifetime because are being broken due to molecular motion (as explained in the paragraph under eqns 1 & 2) and so the correlation function decays away. A plot of the correlation gives the lifetime of the H bonds.