[gmx-users] lifetime of hydrogen bond of bulk water too large

[Calixte, Emvia I]

Hello all, I am finding the lifetime of hydrogen bonds between water molecules for various systems. To compare my values to bulk I have determined bond lifetimes of a 4x4x4 nm3 box of spc/e water. The expected lifetime  should be around 0.27 seconds, however i am obtaining values around 2.1 to 2.3 ps using versions 4.6 and 5.0 beta of gromacs. My simulation is for 30 ns, I collect data every 1ps. The ac function looks fine. I get a good value for number of hydrogen bonds per water molecule of 3.6.  Any help would be appreciated. 

I use g_hbond_mpi -b 2000 -e 10000 -f *.xtc -s *.tpr -n *.ndx -num *.xvg -ac hbacboxsol0M.xvg -dist *.xvg -ang *.xvg -hbn *.ndx 

And my results are

Specify 2 groups to analyze:
Group     0 (         System) has  6540 elements
Group     1 (            SOL) has  6540 elements
Group     2 (             OW) has  2180 elements
Group     3 (            HW1) has  2180 elements
Group     4 (            HW2) has  2180 elements
Select a group: 1
Selected 1: 'SOL'
Select a group: 1
Selected 1: 'SOL'
Calculating hydrogen bonds in SOL (6540 atoms)
Found 2180 donors and 2180 acceptors
Making hbmap structure...done.
Reading frame       0 time 2000.000   
Will do grid-seach on 9x9x9 grid, rcut=0.35
Frame loop parallelized with OpenMP using 8 threads.
Last frame       8000 time 10000.000   
Found 2007112 different hydrogen bonds in trajectory
Found 2174939 different atom-pairs within hydrogen bonding distance
Merging hbonds with Acceptor and Donor swapped
2180/2180
- Reduced number of hbonds from 2007112 to 2007112
- Reduced number of distances from 2174939 to 2174939

Average number of hbonds per timeframe 3927.492 out of 2.3762e+06 possible

++++ PLEASE READ AND CITE THE FOLLOWING REFERENCE ++++
D. van der Spoel, P. J. van Maaren, P. Larsson and N. Timneanu
Thermodynamics of hydrogen bonding in hydrophilic and hydrophobic media
J. Phys. Chem. B 110 (2006) pp. 4393-4398
-------- -------- --- Thank You --- -------- --------

Doing autocorrelation according to the theory of Luzar and Chandler.
ACF 2007112/2007112D
Normalization for c(t) = 0.000253512 for gh(t) = 3.1823e-08

Hydrogen bond thermodynamics at T = 298.15 K
Fitting parameters chi^2 = 0.00497935
Q =          0
--------------------------------------------------
Type      Rate (1/ps) Time (ps)  DG (kJ/mol)  Chi^2
Forward         0.436      2.296       6.588  0.00497935
Backward        0.946      1.057       4.665
One-way         0.416      2.405       6.704
Integral        0.144      6.967       9.340
Relaxation      0.241      4.154       8.058