[gmx-users] strange vdw energy from rerun with GBSA model.

[Da-Wei Li]

Dear Mark and others

I did more tests and thought that it might come from numerical error. The
reasons are

1. If I use .trr file instead of the low precision xtc file, things become
better, ie, I get much less snapshots that has high energy.

2. I supplied -pforce in my mdrun -rerun and found that the high vdw energy
was usually caused by one pair of atoms, whose distance was just very near
the clash zone, so that small error on the coordinates would cause large
energy error. The force is always around 10000.

3. Actually bond length and bond angle energies are also affected. I can
fully reproduce these two energies if I use .trr file in my rerun but will
get several tens of kj/mol error if I use .xtc file, for a protein of size
of 100 AA.


Now the question I still have is whether numerical error can be so large?
The xtc file has a precision of 0.001 nm. Anyway, I will test more by using
double precision Gromacs and define energy groups so that I can compare
energy of protein directly between original MD and rerun.



To Mark only

Thanks. Here it is my script for

 rerun:    mdrun -v -s pbsa.tpr -rerun coor.xtc -e rerun
superpose:  trjconv -s em.tpr -f coor.xtc -o nojump.xtc -pbc nojump  (em.tpr
is generated for energy minimization, protein is in the middle of the box)

rerun .mdp file:

**********************************************************

; Run parameters
integrator    = md        ; leap-frog integrator
nsteps        = 50000000    ; 100 ns
dt        = 0.002            ; 2 fs
; Output control
nstxout        = 500000    ; save coordinates every 1000 ps
nstvout        = 500000    ; save velocities every 1000 ps
nstxtcout    = 5000        ; xtc compressed trajectory output every 1 ps
nstenergy    = 5000        ; save energies every 1 ps
nstlog        = 5000        ; update log file every 1 ps
xtc_grps    = Protein    ; save protein part only
; Bond parameters
continuation    = yes        ; Restarting after NPT
constraint_algorithm = lincs    ; holonomic constraints
constraints    = hbonds    ; all bonds (even heavy atom-H bonds) constrained
lincs_iter    = 1        ; accuracy of LINCS
lincs_order    = 4        ; also related to accuracy
; Neighborsearching
ns_type        = grid        ; search neighboring grid cels
nstlist        = 10        ; 20 fs
rlist        = 0.8        ; short-range neighborlist cutoff (in nm)
rcoulomb    = 0.8        ; short-range electrostatic cutoff (in nm)
rvdw        = 1.0        ; short-range van der Waals cutoff (in nm)
; Electrostatics
coulombtype    = cut-off    ; Particle Mesh Ewald for long-range
electrostatics
pme_order    = 4            ; cubic interpolation
fourierspacing    = 0.12    ; grid spacing for FFT
; Temperature coupling is on
tcoupl        = no        ; modified Berendsen thermostat
tc-grps        = System    ; two coupling groups - more accurate
tau_t        = 0.1        ; time constant, in ps
ref_t        = 300         ; reference temperature, one for each group, in K
; Pressure coupling is on
pcoupl        = no        ; Pressure coupling on in NPT
pcoupltype    = isotropic    ; uniform scaling of box vectors
tau_p        = 2.0        ; time constant, in ps
ref_p        = 1.0        ; reference pressure, in bar
compressibility = 4.5e-5    ; isothermal compressibility of water, bar^-1
; Periodic boundary conditions
pbc        = no        ; 3-D PBC
; Dispersion correction
;DispCorr    = EnerPres    ; account for cut-off vdW scheme
DispCorr    = no
; Velocity generation
gen_vel        = no        ; Velocity generation is off



; IMPLICIT SOLVENT ALGORITHM
implicit_solvent         = GBSA
gb_algorithm             = OBC
nstgbradii               = 1
rgbradii                 = 0.8
gb_epsilon_solvent       = 80
gb_saltconc              = 0
gb_obc_alpha             = 1
gb_obc_beta              = 0.8
gb_obc_gamma             = 4.85
gb_dielectric_offset     = 0.009
sa_algorithm             = Ace-approximation
sa_surface_tension       = 2.25936

***************************************************************************************

Thanks all.

dawei
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