[gmx-users] why the energy items in gromacs is different from charmm?

[wliu]

Hi,

I have a following issue:
     I use the force field CHARMM22-CMAP (that is charmm27-CMAP) in 
gromacs to calculate the chignolin.pdb, I want to know the energy items 
of just this one conformation. I choose to run a very simple md, just 
one step. my input file writes as follows:

md.mdp
emtol                    = 10
emstep                   = 0.001
niter                    = 1
fcstep                   = 0
nstcgsteep               = 1000
nbfgscorr                = 10
rtpi                     = 0.05
nstxout                  = 0
nstvout                  = 0
nstfout                  = 0
nstlog                   = 100
nstcalcenergy            = -1
nstenergy                = 10
nstxtcout                = 100
xtc-precision            = 1000
energygrps               = System


And I also use CHARMM with the same force field CHARMM22-CMAP, but the 
enerCHARMM.pl command to calculate chignolin's energy items. But I found 
that their results are quite different. I'd like to list below:

                               charmm22     gromacs/md22
chignolin(kcal/mol)
angles                         20.9848           /
bonds                          22.7760        195.141
cmap                           -0.0595         -0.0684
dihedrals                      46.0533           /
elec                           19.5690           /
impropers                       0.0002          0.014
total                      453415.3682     201520.813
ureyb                           2.7128         38.779
vdwaals                    453303.3316           /
proper dihedral                  /             46.478
LJ-14                            /             82.409
Coulomb-14                       /            650.335
LJ (SR)                          /         201106.699
Coulomb (SR)                     /           -598.990


torsion                        45.994          46.4236
                             =  -0.0595     =   -0.0684
                              + 46.0533      +  46.478
                              +  0.0002      +   0.014               ( I 
have Unified their unit to kcal/mol)

I want to know why their results shown such a difference? I also use 
step=0 and minimization method to try, but nearly the same result. 
Theoretically，Two different MD programs calculating one same structure 
should give out the results which differ no more than 0.01kcal/mol, but, 
this makes me confused. So could you please help me find where the 
problem is or the reasons?
Thank you!

Looking forward to your kind reply！


Sincerely yours,
Wei Liu，

wliu at itcs.ecnu.edu.cn
East China Normal University
ShangHai，china，200062