Force field for fully flexible water

[nadler]

Hi all

Recently I have seen a force field that allows the calculation of
fully flexible water molecules, which I'd like to use (X.B. Zhang in
Fluid Phase Equilibria 262 (2007) p. 210). The problem is that when
running a test, the molecules just "explode" and I did not find out
why. The input is as follows. The uncommented lines are resulting from
an attempt to apply restraints, which did not work as well. The
molecules again "exploded". Can anyone help me with that? Thanks!

Cheers, Roger.


&FORCE_EVAL
  METHOD FIST
  &MM
    &FORCEFIELD
#      &BEND
#        ATOMS H O H
#        K 0.
#        THETA0 1.8
#      &END BEND
#      &BOND
#        ATOMS O H
#        K 0.
#        R0 1.8
#      &END BOND
      &BEND
        ATOMS H O H
        KIND HARMONIC
        K [kjmol*deg^-2] 353.0
        THETA0 [deg] 109.4712
      &END BEND
      &BOND
        ATOMS O H
        KIND HARMONIC
        K [kjmol*angstrom^-2] 1480.0
        R0 [angstrom] 0.9611
      &END BOND
      &CHARGE
        ATOM O
        CHARGE -0.8476
      &END CHARGE
      &CHARGE
        ATOM H
        CHARGE 0.4238
      &END CHARGE
      &NONBONDED
        &LENNARD-JONES
          atoms O O
          EPSILON [kjmol] 0.6287
          SIGMA 3.1169
          RCUT 10.0
        &END LENNARD-JONES
        &LENNARD-JONES
          atoms O H
          EPSILON [kjmol] 0.142723
          SIGMA 2.04845
          RCUT 10.0
        &END LENNARD-JONES
        &LENNARD-JONES
          atoms H H
          EPSILON [kjmol] 0.0324
          SIGMA 0.98
          RCUT 10.0
        &END LENNARD-JONES
      &END NONBONDED
    &END FORCEFIELD
    &POISSON
      &EWALD
        EWALD_TYPE spme
        ALPHA .35
        GMAX 25
        O_SPLINE 4
      &END EWALD
    &END POISSON
  &END MM
  &SUBSYS
    &CELL
      ABC 24.955 24.955 24.955
      UNIT ANGSTROM
    &END CELL
    &COORD
   O  22.9836691917178939 6.93183557997552668 0.754607553842305823
H2O
   H  23.1493887587765457 6.38145839360815526 1.57291188465716791
H2O
   H  23.8522379765498265 7.11351376960166615 0.293542155628028989
H2O
   O  9.55748115579216773 10.1625527712466361 -12.8321807856825103
H2O
   H  9.48825863164111638 10.8031673228011336 -13.5969168437373611
H2O
   H  8.66207900403075648 9.75244702889359871 -12.6587787478834102
H2O
   O  19.8436465738885985 10.0378451904347870 9.93036518412813862
H2O
   H  19.1357369126550836 9.37699136789161081 9.68109397285001450
H2O
   H  19.9908902622388958 10.0115791753817565 10.9191166153261410
H2O
    &END COORD
#    &COLVAR
#      &DISTANCE
#        ATOMS 1 2
#      &END DISTANCE
#    &END COLVAR
#    &COLVAR
#      &DISTANCE
#        ATOMS 1 3
#      &END DISTANCE
#    &END COLVAR
#    &COLVAR
#      &ANGLE
#        ATOMS 2 1 3
#      &END ANGLE
#    &END COLVAR
  &END SUBSYS
&END FORCE_EVAL
&GLOBAL

  PROJECT water
  RUN_TYPE md
&END GLOBAL
&MOTION
#  &CONSTRAINT
#    &COLLECTIVE
#      TARGET [angstrom]  0.9611
#      MOLNAME H2O
#      COLVAR 1
#      &RESTRAINT
#        K [kjmol*angstrom^-2] 1480.0
#      &END RESTRAINT
#    &END COLLECTIVE
#    &COLLECTIVE
#      TARGET [angstrom]  0.9611
#      MOLNAME H2O
#      COLVAR 2
#      &RESTRAINT
#        K [kjmol*angstrom^-2] 1480.0
#      &END RESTRAINT
#    &END COLLECTIVE
#    &COLLECTIVE
#      TARGET [deg]  109.4712
#      MOLNAME H2O
#      COLVAR 3
#      &RESTRAINT
#          K   [kjmol*deg^-2]  353.0
#      &END
#    &END COLLECTIVE
  &END CONSTRAINT
  &MD
    ENSEMBLE NVT
    STEPS 100
    TIMESTEP 1.0
    TEMPERATURE 298
    &THERMOSTAT
      REGION MOLECULE
      &NOSE
        LENGTH 3
        YOSHIDA 3
        TIMECON 1000
        MTS 2
      &END NOSE
    &END
  &END MD
&END MOTION