Intrinsic-reaction-coordinate calculations

The potential energy profiles along intrinsic reaction coordinate (IRC) can be computed via d method of Hratchian and Schlegel^[1]. The algorithm starts from transition state and propagates the system via damped velocity Verlet algorithm. The damping is realized via rescaling the velocity vector to a constant value ( $v_0$ ) after each propagation step. At the same time, the time step is adaptively changed so as to ensure that the trajectory generated by the algorithm does not differ from true IRC by more than the predefined tolerance factor $\Delta_0$ . As an input, the structure of well relaxed transition state and the direction of unstable vibration mode must be provided. For that purpose, a CONTCAR file from an improved-dimer-method calculation converged with a tight relaxation criterion (e.g., EDIFFG =-0.005) can be used. To obtain a complete energy profile along IRC connecting two stable states, two independent calculations with positive (IRC_DIRECTION =1) and negative (IRC_DIRECTION =-1) initial displacement along the direction of the unstable mode must be performed.

The following parameters can be modified to affect the performance of the method:

IRC_DIRECTION direction of the initial displacement (-1|1 – negative|positive)
IRC_STOP the number of steps the energy must monotonously increase before the algorithm terminates. In order to avoid a premature terminations, especially close to transition states., e.g., due to a numerical noise, IRC_STOP should always be greater than 1.
IRC_DELTA0 the tolerance factor $\Delta_0$ in Å – the smaller value, the closer the computed trajectory follows the true IRC (but the more DFT steps is needed)
IRC_MINSTEP specifies the lower limit for the time step in fs
IRC_MAXSTEP specifies the upper limit for the time step in fs
IRC_VNORM0 the value of $v_0$ in Å/fs

Mind: This method is presently available only for fixed cell shape (i.e., ISIF = 2) simulations.

Mind: The calculation must be initialized from a very well relaxed transition state (EDIFFG = -0.005 or less in absolute value).

Practical example

↑ H. P. Hratchian and H. B. Schlegel, Following Reaction Pathways Using a Damped Classical Trajectory Algorithm, J. Phys. Chem. A 106, 165 (2002).

[hratchian:jpc:2002-1] H. P. Hratchian and H. B. Schlegel, Following Reaction Pathways Using a Damped Classical Trajectory Algorithm, J. Phys. Chem. A 106, 165 (2002).

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