HCN: transition state search with the CINEB method

Sample directory: adf/HCN_CINEB/

This example demonstrates the use of the Nudged Elastic Band method in ADF for finding a transition state of the HCN isomerisation reaction. A shell script used to run the example calculation is shown below:

$ADFBIN/adf <<eor
TITLE Test of the CI-NEB method

SYMMETRY C(S)

NOPRINT SCF SFO
UNITS
    length Angstrom
    angle Degree
END

ATOMS
    1.C         0.000000    0.000000    0.000000
    2.N         XN          0.000000    0.000000
    3.H         XH          YH          0.000000
END

GEOVAR
XN      1.180  1.163
XH      2.196  1.831 1.006 0.105 -0.718 -1.078
YH      0.000  0.799 1.122 1.163  0.813  0.000
END

BASIS
END

GEOMETRY
  CINEB      9
  iterations 150
  OPTIM selected
  converge grad=0.001
  nebspring 1 0.06
END

integration 4.0

SCF
  Convergence 0.00000001
END

eor

A few important points to note:

• Symmetry is set to C_s explicitly because all images must have the same symmetry but symmetry of the reaction products is higher, C_∞. Thus, it is necessary to lower the overall symmetry to match the lowest.
• In the GEOVAR section, there are six values specified for the coordinates of the hydrogen atom. Thisi is necessary in order to bring the initial guess for the reaction path closer to the final result. Moreover, if only two values for YH were specified (0.0 and 0.0) then the hydrogen atom would be "dragged" along the C-N bond leading to unrealistic geometries and, eventually, to a failure.
• In the GEOMETRY section of the input, the number of NEB images is set to 9; the convergence criterion is lowered to 0.001; optimization of only the coordinates specified in the GEOVAR block is requested;
• the NEB spring parameter is set to a constant (energy-independent) value of 0.06 a.u.. The choice of the parameter depends on the stiffness of the bonds involved in the reaction and the value of the parameter should, in principle, be of about the same magnitude as the Hessian eigenvalue for the coordinates participating in the reaction.