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Improved dimer method

The dimer method [61] is a technique for the optimization of transition states. In VASP, the method improved by Heyden et al. [62] (IDM) is implemented, detailed presentation of the method can be found in Ref. [62]. Algorithm for IDM consists of the following cyclically repeated steps:

The method is invoked by setting IBRION=44 (see Sec. 6.22).

Furthermore, user must specify direction of the unstable mode. Corresponding 3N dimensional vector is defined in the POSCAR file after the lines with atomic coordinates and a separating blank line. Note that the dimer direction is automatically normalized, i.e. the norm of the dimer axis defined by user is irrelevant. Example of POSCAR file for simulation with dimer method:

ammonia flipping
1.
6. 0. 0.
0. 7. 0.
0. 0. 8.
H N
3 1
cart
       -0.872954        0.000000       -0.504000        ! coordinates for atom 1
        0.000000        0.000000        1.008000
        0.872954        0.000000       -0.504000
        0.000000        0.000000        0.000000        ! coordinates for atom N
       ! here we define trial unstable direction:
        0.000001    0.522103   -0.000009        ! components for atom 1
       -0.000006    0.530068    0.000000
       -0.000005    0.522067   -0.000007
        0.000001   -0.111442    0.000001        ! components for atom N

As in the other structural optimization algorithms in VASP, convergence is controlled through the EDIFFG tag.

Experienced users can affect the performance of the dimer method by modifying the numerical values of the following parameters:

Important information about the progress of optimization is written in the file OUTCAR after the expression 'DIMER METHOD'. In particular, it is useful to check the curvature along the dimer direction, which should be a negative number (long sequence of positive numbers usually indicates that the algorithm fails to converge to the correct transition state).

IMPORTANT NOTE: The current implementation does not support lattice optimizations (ISIF$ >$2) and can be used only for the relaxation of atomic positions.



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N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at