Editing IDM-IRC (section)

== Intrinsic Reaction Cooordinate (IRC) ==

The current implementation does not support lattice optimization and can be used only for atomic relaxations (<TT>ISIF=2</TT>).

<p align="center" style="width:100%">
{| border="1"
 |+ '''Flags and parameters'''
 |- 
 | <TT>IBRION=40</TT>
 | # perform IRC search using the DVV method
 |- 
 | <TT>DVVEHISTORY=5</TT>
 | # number of subsequent increasing energy steps taken before simulation is terminated
 |- 
 | <TT>DVVDELTA0=1.5e-3</TT>
 | # the error tolerance &#916;<sub>0</sub>
 |- 
 | <TT>DVVVNORM0=0.01</TT>
 | # magnitude of the velocity vector v<sub>0</sub>
 |- 
 | <TT>DVVMINPOTIM=0.025</TT>
 | # lower limit for the time step &#916;''t''
 |- 
 | <TT>DVVMAXPOTIM=3.0</TT>
 | # upper limit for the time step &#916;''t''
 |- 
 | <TT>DVVMINUS=.FALSE.&#124;.TRUE.</TT>
 |  # take positive|negative direction of the initial velocity vector
 |}
</p>

The method is described in detail in Ref.(3). Velocity verlet algorithm is used to drive the system from a '''converged''' saddle point into a nearby minimum along intrinsic reaction coordinate (IRC):

: ''  x<sub>i</sub> = x<sub>i-1</sub> + v<sub>i-1</sub>&#916;t<sub>i</sub> + &#189;a<sub>i-1</sub>&#916;t<sub>i</sub><sup>2</sup>  ''
: ''  v<sub>i</sub> = v<sub>i-1</sub> + &#189;(a<sub>i-1</sub>+a<sub>i</sub>)&#916;t<sub>i</sub>  .''

Direction of atomic velocities (i.e. vector of negative curvature) must be defined in POSCAR. Note that if DVVMINUS is set to =.FALSE., opposite direction of vector defined in POSCAR is used to initiate atomic velocities. In order to keep intermediate structures close to IRC, atomic velocities are damped such as the norm of velocity vector is constant:

: ''  &#124;v<sub>i</sub>&#124; = v<sub>0</sub>  .''

The size of integration step is varied during the simulation such as to ensure optimal performance:

: ''  &#916;t<sub>i+1</sub> = &#916;t<sub>i</sub>&#124;&#916;<sub>0</sub>/&#916;<sub>i</sub> &#124;<sup>1/3</sup> ,''

&#916;t<sub>i</sub> is constrained to interval <TT>DVVMINPOTIM</TT> < &#916;t<sub>i</sub> < <TT>DVVMAXPOTIM</TT>.
Output (in particular value of intrinsic reaction coordinate (IRC) and corresponding potential energy) is written into <TT>OUTCAR</TT> file after key words "<TT>DAMPED VELOCITY VERLET ALGORITHM:</TT>"

(3) H.P. Hratchian and H.B. Schlegel. ''Following reaction pathways using a damped classical trajectory algorithm.'' J. Phys. Chem. A, '''106(1)''':165–169, 2002. [http://dx.doi.org/10.1021/jp012125b]