Editing IDM-IRC (section)

== Brief Hands-on Guide for IDM+IRC ''(by David)'' ==

'''Finding a transition state (TS) using the improved dimer method (IDM)''' 

:1. ''Suggest the geometry of the TS''.
Do not care much about the structure of initial state (IS) and final state (FS) of the reaction as it is for (CI-)NEB calculations, simply stretch out bond(s) to be broken or shorten bond(s) to be formed, by hand in a visualizer like [[P4V]]. 
Of course, you can create a reasonable TS guess using tools like <TT>nebmake.pl</TT> (from VTST tools) from IS and FS, or re-use a TS result structure from (CI-)NEB calculation. 

:2. ''Run a frequency job for the TS guess'', 
i.e. ensure <TT>IBRION=5</TT>, <TT>NFREE=2</TT>, very small <TT>POTIM=0.02</TT> and <TT>NWRITE=3</TT> (important!) in your INCAR file. Use low ''k''-point grid in <TT>KPOINTS</TT>, because there is no need for high accuracy at this step.
Hopefully you have a "hard" imaginary vibration mode (no matter that there are some very soft ones). Optionally, you can check the modes using [[nfreq.sh]] script in MOLDEN. Now, extract the displacement vector of the imaginary mode, preferably using [[get-dimer-dir.sh]] script.

:3. ''Submit the IDM job''. 
The most important keyword switching on the IDM method is <TT>IBRION=44</TT> tag. Don't worry to setup the <TT>NSW</TT>-tag in the INCAR file to a very high number such as 1000+ or so, the calculation may need a lot of ionic steps.  The input files are the same as for usual relaxation run (INCAR+KPOINTS+POSCAR+POTCAR), the POSCAR containing the eigenvector. Use VASP 5.2.11 (or later) binary.
Sometimes, it is needed to refine the TS structure ''via'' resubmitting for classical optimization with <TT>IBRION=1</TT> and small <TT>POTIM=0.1</TT>.

:4. ''Check the TS you've obtained'', 
i.e. run a frequency job (<TT>IBRION=5</TT>) and check that you have 1 "hard" imaginary vibration mode. Well done!
Afterwards, you may want to find the corresponding IS and TS (incl. full energy profile) using the IRC method (''vide infra'').

'''Finding the initial state (IS) and final state (FS) using the improved dimer method (IRC)''' 

:1. ''Slide down the energy saddle from the TS''. 
Use the CONTCAR from a preceding IDM calculation as POSCAR (contains the dimer vector inside) for the two IRC runs. The most important INCAR keyword switching on the IRC method is <TT>IBRION=40</TT> tag. Furthermore, you need 2 directories, in which the INCARs differ by the setting for sliding down the forward (<TT>DVVMINUS=.FALSE.</TT>) and reverse (<TT>DVVMINUS=.TRUE.</TT>) direction from the saddle point (the TS).

:2. ''Explore the full energy path''. 
Put the energies of the forward and reverse runs together and enjoy your full energy trajectory by pasting the output of the script [[get-irc-path.sh]] to your favourite graph-plotting utility.