remarks file toppar/water.param 
remarks   water parameters for structure determination 
remarks

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 checkversion 1.2


bond ht   ot     450.0       0.9572    ! from TIPS3P geometry 
bond ht   ht       0.0       1.5139    ! from TIPS3P geometry 

angle ht   ot   ht      55.0     104.52    ! FROM TIPS3P geometry


{* nonbonding parameter section *}
{* ============================ *}
!!
!  This uses a new form of the REPEL function:
!    fVDW(R) =  RCON *( Rmin ^ IREX - R  ^ IREX ) ^ REXP
!
!  PROLSQ uses a function of the form:
!    fVDW(R) =  (1 / 0.5) ^ 4 * ( Rmin - R  ) ^ 4
!
!  The epsilon values are arbitrary since the repel function does not depend
!   on epsilon.  The sigma values come from converting the Van der Waals 
!   radii of the PROLSQ program into sigma values using the formula:
!     Rmin = sigma * 2 ^ (1/6)
!   Note:  Prolsq decrements Van der Waals radii for non-bonded contacts
!           that involve torsion angles (1:4 contacts) by .30 A, and
!           hydrogen bonds (X...Y) by .2 A (X-H...Y) by .9.  The former
!           decrement is accomplished in CNS by using the
!           1-4 nonbonded terms.  The latter decrement is accomplished by
!           decreasing the van der Waals radius of hydrogens by 0.8 
!           and that of O and N by 0.1 A.
!

 NBONds
  CUTNB=7.0   WMIN=1.5
  REPEl = 1.0          
  REXPonent = 4
  IREXponent = 1
  RCONst = 16.0
  TOLErance = 0.5      NBXMOD = 5
  ctonnb=5.5 ctofnb=6.0 {* for consistency only, not needed for repel *}
END

!                  type    2 x van der Waals radius     correction applied for hbond
evaluate ($vdw_radius_O=        2.9)                             {-0.1}
evaluate ($vdw_radius_HH=       1.6)                             {-0.8}

{ convert radii into sigmas }
evaluate ($vdw_radius_O   = $vdw_radius_O    / 2^(1/6))
evaluate ($vdw_radius_HH  = $vdw_radius_HH   / 2^(1/6))

{ compute 1-4 sigmas }
evaluate ($vdw_radius14_O   = $vdw_radius_O    -0.3/ 2^(1/6))
evaluate ($vdw_radius14_HH  = $vdw_radius_HH   -0.3/ 2^(1/6))

evaluate ($vdw_eps=0.1)

!
! sigma= vdw radius / 2 ^ (1/6)
!
!                  eps     sigma              eps(1:4) sigma(1:4)
!                  (kcal/mol) (A)             (kcal/mol) (A)
!                   --------------------------------------------------

NONBonded  HT      $vdw_eps $vdw_radius_HH   $vdw_eps $vdw_radius14_HH
!
NONBonded  OT      $vdw_eps $vdw_radius_O    $vdw_eps $vdw_radius14_O
!
 
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