[gmx-users] minimization force criteria

[tarak karmakar]

Dear All,

While running minimization I imposed the the condition for the
minimization as to be converged only at Fmax < 10 . But I got the
following


Stepsize too small, or no change in energy.
Converged to machine precision,
but not to the requested precision Fmax < 10

Double precision normally gives you higher accuracy.
You might need to increase your constraint accuracy, or turn
off constraints alltogether (set constraints = none in mdp file)

Steepest Descents converged to machine precision in 15318 steps,
but did not reach the requested Fmax < 10.
Potential Energy  = -3.3397742e+06
Maximum force     =  9.8232233e+02 on atom 6057
Norm of force     =  3.6321018e+00


Then according to the above instruction I increased the stepsize up to
0.5 [ I didn't use any constraint during minimization] but got the
same. So do I need to increase the stepsize more or is there other
procedure to get to the desired convergence criteria ?
The minimization .mdp file is as follows


; 7.3.2 Preprocessing
;define                  = -DFLEXIBLE    ; defines to pass to the preprocessor

; 7.3.3 Run Control
integrator              = steep         ; steepest descents energy minimization
nsteps                  = 100000         ; maximum number of steps to integrate

; 7.3.5 Energy Minimization
emtol                   = 10            ; [kJ/mol/nm] minimization is
converged when max force is < emtol
emstep                  = 0.01           ; [nm] initial step-size
nstcgsteep              = 10            ;  freq of performing one
steepest descent step

; 7.3.8 Output Control
nstxout                 = 500           ; [steps] freq to write
coordinates to trajectory
nstvout                 = 500           ; [steps] freq to write
velocities to trajectory
nstfout                 = 500           ; [steps] freq to write forces
to trajectory
nstlog                  = 100           ; [steps] freq to write
energies to log file
nstenergy               = 100           ; [steps] freq to write
energies to energy file
energygrps              = System        ; group(s) to write to energy file

; 7.3.9 Neighbor Searching
nstlist                 = 1             ; [steps] freq to update neighbor list
ns_type                 = grid          ; method of updating neighbor list
pbc                     = xyz           ; periodic boundary conditions
in all directions
rlist                   = 0.8           ; [nm] cut-off distance for
the short-range neighbor list

; 7.3.10 Electrostatics
coulombtype             = PME           ; Particle-Mesh Ewald electrostatics
rcoulomb                = 0.8           ; [nm] distance for Coulomb cut-off

; 7.3.11 VdW
vdwtype                 = cut-off       ; twin-range cut-off with
rlist where rvdw >= rlist
rvdw                    = 0.8           ; [nm] distance for LJ cut-off
DispCorr                = Ener          ; apply long range dispersion
corrections for energy

; 7.3.13 Ewald
fourierspacing          = 0.12          ; [nm] grid spacing for FFT
grid when using PME
pme_order               = 4             ; interpolation order for PME, 4 = cubic
ewald_rtol              = 1e-5          ; relative strength of
Ewald-shifted potential at rcoulomb


Thanks,

Tarak