[gmx-users] g_tune_pme big standard deviation in perf.out output

[WU Yanbin]

Dear GMXers,

I'm simulating a SPC/E water box with the size of 4nm by 4nm by 4nm. The
command "g_tune_pme" was used to find the optimal PME node numbers, Coulomb
cutoff radius and grid spacing size.

The following command is used:
g_tune_pme -np 24 -steps 5000 -resetstep 500 ...
rcoul=1.5nm, rvdw=1.5nm, fourierspacing=0.12

The simulation is done with no error. Below is the output:
---------------------------
Line tpr PME nodes  Gcycles Av.     Std.dev.       ns/day        PME/f    DD
grid
   0   0   12          2813.762      187.115        9.604        0.361
4   3   1
   1   0   11          2969.826      251.210        9.112        0.510
13   1   1
   2   0   10          2373.469      154.005       11.385        0.445
2   7   1
   3   0    9          2129.519       58.132       12.665        0.601
5   3   1
   4   0    8          2411.653      265.233       11.248        0.570
4   4   1
   5   0    7          2062.770      514.023       13.490        0.616
17   1   1
   6   0    6          1539.237       89.189       17.547        0.748
6   3   1
   7   0    0          1633.318      113.037       16.548          -
6   4   1
   8   0   -1(  4)     1330.146       32.362       20.276        1.050
4   5   1
---------------------------

The optimal -npme is 4.

It seems to me that the "Std. dev" is too huge.
Can anyone tell me the meaning of "Gcycles Av." and "Std. dev" and their
relations to the accuracy of "ns/day"?

Another question:
I tried
g_tune_pme -np 24 -steps 1000 -resetstep 100 ... (the default value of
g_tune_pme)
rcoul=1.5nm, rvdw=1.5nm, fourierspacing=0.12

The optimal -npme is 6, different from "-npme=4" as obtained with big
"-nsteps".
Should I increase "-nsteps" even more to get better estimate, or what else
parameters should I try?

Do let me know if the questions are not made clear.
Thank you.

Best,
Yanbin
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