[gmx-users] g_hbond

[Chinmay Das]

Hi,
  I am having trouble understanding the criteria for hydrogen bond and
how it is implemented in g_hbond. Defining hydrogen bond for O1--H..O2
by requiring distance(O1, O2) < 0.35 nm and angle(O1-H, H-O2) < 30
degrees, my own code consistently give lower number of hydrogen bonds
than g_hbond. But my results match if I count the hydrogen bonds
plotted with vmd using the same criteria. (To test this I created a
small system of water molecules at the center of a large box - so
periodic boundary condition is not the reason.)
Can some one help?

Some details: I placed randomly oriented 8 SPC water on the vertices
of a cube with sides of size 0.3, starting at {x,y,z}={1.5,1.5,1.5}
and set the box length to be {4, 4, 4}. A very short NVT run at 30K
orients the water but doesn't move them much. vmd shows that the
configuration has 3 hydrogen bonds and my calculation also picks up
the same triplets and identifies as hydrogen bond. However, g_hbond
-hbn picks up 4. The hbonds_SOL entry in hbond.ndx have two of the
correct entries and two entries which seem wrong.

hbond.ndx entry :
[ hbonds_SOL ]
      1      2     13
     10     11     22
     16     17     13
     22     23     16
What I find :
1-2 -> 13
16-17 -> 13
22-24 -> 16

And the configuration file :
Pure Water
   24
    1SOL     OW    1   1.494   1.496   1.495 -0.2407 -0.3859 -0.4517
    1SOL    HW1    2   1.593   1.511   1.498 -0.6476  3.2879  3.3372
    1SOL    HW2    3   1.470   1.416   1.551  0.4456  0.5879  1.2955
    2SOL     OW    4   1.506   1.503   1.801  0.3278  0.0764  0.1469
    2SOL    HW1    5   1.586   1.482   1.857  0.3064  1.6146  0.8017
    2SOL    HW2    6   1.435   1.434   1.817  0.1439  0.6902  2.4044
    3SOL     OW    7   1.499   1.810   1.507 -0.1617  0.6516  0.4652
    3SOL    HW1    8   1.446   1.767   1.579 -0.1540 -3.2274 -1.5277
    3SOL    HW2    9   1.439   1.833   1.430  0.5590 -2.0347 -1.0463
    4SOL     OW   10   1.499   1.802   1.796 -0.0922  0.1504 -0.2160
    4SOL    HW1   11   1.501   1.717   1.849 -2.8903 -1.0982 -1.9099
    4SOL    HW2   12   1.587   1.815   1.750  1.3885 -0.2196  2.3356
    5SOL     OW   13   1.798   1.491   1.501 -0.1855 -0.6306 -0.0502
    5SOL    HW1   14   1.856   1.410   1.508  2.6216  1.0980 -1.5287
    5SOL    HW2   15   1.813   1.535   1.412  0.8590  3.2110  1.8097
    6SOL     OW   16   1.804   1.499   1.803  0.1303 -0.0398  0.1705
    6SOL    HW1   17   1.771   1.492   1.709  2.6087  3.3512 -1.1639
    6SOL    HW2   18   1.865   1.423   1.824 -0.0578 -0.5609 -1.1170
    7SOL     OW   19   1.806   1.799   1.496  0.4549 -0.1262 -0.0397
    7SOL    HW1   20   1.762   1.879   1.537 -3.4166  1.6216 -6.2709
    7SOL    HW2   21   1.754   1.717   1.521 -3.2644  1.1647 -2.6085
    8SOL     OW   22   1.796   1.797   1.802 -0.2321 -0.1817  0.1797
    8SOL    HW1   23   1.830   1.847   1.722  3.7237 -1.1847  0.9954
    8SOL    HW2   24   1.808   1.699   1.788 -2.2397 -0.5892  0.9430
   4.00000   4.00000   4.00000

Thanks in advance.
Chinmay