Editing IDM-IRC (section)

== Improved Dimer Method (IDM) ==

The current implementation does not support lattice optimisation 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=44</TT>
 | # invokes relaxation with the dimer method
 |- 
 | <TT>FINDIFF=1&#124;2</TT>
 | # forward (1) of central (2) differences formula for numerical differentiation
 |- 
 | <TT>DIMER DIST=0.01</TT>
 | # step for numerical differentiation (&#197;)
 |- 
 | <TT>STEP SIZE=0.01</TT>
 | # trial step size for energy minimisation(&#197;)
 |- 
 | <TT>STEP MAX=0.1</TT>
 | # maximal step size for energy minimisation (&#197;)
 |- 
 | <TT>MINROT=0.01</TT>
 | # minimal rotation of dimer (rad.)
 |}
</p>

The method is described in detail in Ref.(2). The only input which has to be defined by user is a ''3N''-dimensional vector defining direction of negative curvature on potential energy surface. It must be specified in <TT>POSCAR</TT> on place of ionic velocities (see documentation for the <TT>POSCAR</TT> file). Note that this vector is automatically normalised so it is the only direction that really matters. Output including maximal gradient in the current step, curvature along the dimer direction and the angle through which the dimer is rotated is written in <TT>OUTCAR</TT> after key words "<TT>DIMER METHOD</TT>".

(1) G. Henkelman and H. Jónsson. ''A dimer method for finding saddle points on high dimensional potential surfaces using only first derivatives.'' J. Chem. Phys., '''111(15)''':7010–7022, 1999. [http://dx.doi.org/10.1063/1.480097]

(2) A. Heyden, A.T. Bell, and F.J. Keil. ''Efficient methods for finding transition states in chemical reactions: Comparison of improved dimer method and partitioned rational function optimization method.'' J. Chem. Phys., '''123(22)''':224101, 2005. [http://dx.doi.org/10.1063/1.2104507]