ADD_STR_TORS

NAME
ADD_STR_TORS - Add a harmonic bending potential between three target atoms.

SYNOPSIS
ADD_STR_TORS    iat1    iat2     iat3      iat3    k    $\theta_0$    [ $\theta_{\tau}$]

DESCRIPTION
This command can be used to impose an additional (harmonic) torsional constraint between atoms iat1, iat2, iat3 and iat4 of the solute. The axis of the torsion is defined by the atoms iat2, iat3. The numeric order of the solute atom indices iat1, iat2, iat3, iat4 is that specified in the topology file (see Sec. 10.3). The added torsional potential has force constant k (in Kcal/mol/rad$^2$) and equilibrium dihedral angle $\theta_0$ (in degrees). If $\theta_\tau$ is also specified, then the added torsional potential is time dependent and $\theta_{\tau}$ is the equilibrium dihedral angle after the steering time $\tau$ (see STEER(&RUN)) command for the definition of the steering time in a SMD simulation)

WARNINGS
If the chosen $\theta_0$ is very different from the actual value of the dihedral angle at time 0, a very large force is experienced by the atoms in involved in the added bending and the simulation may catastrophically diverge after few steps.

EXAMPLES

• Example 1.
  

   
  ADD_STR_TORS  1 50 70 104  400.  60.0
  

• Example 2.
  

   
  &POTENTIAL 
    ...
    ADD_STR_TORS  1 50 70 104  400.  60.0    90.0
   ..
  &END
  ....
  ...
  &RUN
     ...
     STEER 10000.  50000. 
     ...
  &END
  

In the first example a torsional constraint is imposed between atom 1, atom 50, atom 70 and atom 104 of the solute. In the second example a time-dependent driving potential is applied to the same atoms of the solute. The equilibrium dihedral angle of such harmonic driving potential move at constant velocity in $\tau=40$ ps (starting at t=10 ps ) between $\theta_0=60$ and $\theta_\tau=90$ degrees.