[gmx-users] Bonds and atom-pairs in g_hbond
Andrew DeYoung
adeyoung at andrew.cmu.edu
Mon Jun 11 23:30:54 CEST 2012
Hi,
I am analyzing hydrogen bonds using g_hbond. I have selected two
non-overlapping groups; one is hydroxyl groups (OH, where O is the donor and
H is the hydrogen), and the other is oxygens on PO4^3- (where F is the
acceptor). I am using the switch -noda, which tells the program to take the
-r value as the Hydrogen-Acceptor distance (here, H-O), rather than the
Donor-Acceptor distance (here, O-O). My trajectory is 10 ns long.
In the output after the calculation, I see the following:
Found 233 different hydrogen bonds in trajectory
Found 224 different atom-pairs within hydrogen bonding distance
Can you please help me to understand the terms "hydrogen bond" and
"atom-pair within hydrogen bonding distance" conceptually?
I think that in order for a hydrogen and an acceptor to be considered
hydrogen bonded, two criteria must be satisfied: (1) the distance (in this
case, the distance between the hydrogen and the acceptor, since I am using
-noda) must be less than the cutoff distance -r, and (2) the
Acceptor-Donor-Hydrogen angle must be less than the cutoff angle -a. So
distance and angle are the two criteria that must be met, according to
g_hbond and most definitions of hydrogen bonding.
So, apparently, g_hbond found 233 hydrogen bonds satisfying both the
distance and angle criteria.
But, g_hbond "found 224 different atom-pairs within hydrogen bonding
distance." My guess is that this means that there are 224 atom pairs
satisfying the distance criterion (i.e., Hydrogen-Acceptor pairs within the
cutoff distance, -r). But if my interpretation of the term "atom-pair
within hydrogen bonding distance" is correct, how can there be 9 _fewer_
atom pairs that satisfy only the distance criterion (and not the angle
criterion), than those that satisfy _both_ the distance criterion _and_ the
angle criterion?
Probably, I have the wrong pictures of the terms "hydrogen bond" and
"atom-pair within hydrogen bonding distance" in my head.
Thank you for your time!
Andrew DeYoung
Carnegie Mellon University
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