Intrinsic-reaction-coordinate calculations

The potential energy profiles along intrinsic reaction coordinate (IRC) can be computed via d method of Hratchian and Schlegel. The algorithm starts from transition state and propagates the system via damped velocity Verlet algorithm. The damping is realized via rescaling the velocity vectorto a constant value ($v_0$) after each propagation step.

• IRC_DIRECTION direction of the initial displacement (-1|1 – negative|positive)
• IRC_STOP = 20 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 = 0.0015 the desired error – the smaller value, the closer the computed trajectory follows the true IRC (but the more DFT steps is needed)
• IRC_MINSTEP = 0.0250
• IRC_MAXSTEP = 3.0000
• IRC_VNORM0 = 0.0020 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).