Intrinsic reaction coordinates

A tool that have proved valuable in the study of molecular dynamics is the use of steepest descent-based algorithms for following a molecular reaction from a transition state towards a minimum. One of the most successful ways of doing this is the Intrinsic Reaction Coordinate (IRC) approach [37]. The IRC is calculated by taking small steps along the negative gradient in a mass-weighted coordinate system. In DALTON, the size of the step is adjusted with the use of the trust-region based algorithm. In order to get a sufficiently accurate potential energy surface, rather small steps must be taken, and the default trust radius is thus reset to 0.020 when an IRC calculation is being done.

In many respects the input to an IRC calculation is very similar to the input for a trust-region image optimization, and a typical input would look like:

**DALTON INPUT
.WALK
.MAX IT
 150
*WALK
.IRC
 1
**WAVE FUNCTIONS
.HF
**END OF DALTON INPUT

Most of this input should now be self-explanatory. The request for an Intrinsic Reaction Coordinate calculation is done by using the keyword .IRC. On the next line there is a positive or negative integer indicating in which direction the reaction should proceed. It is, however, not possible to determine in advance which reaction path a given sign is connected to, and the calculation should therefore always be checked after a few iterations in order to ensure that the reaction proceeds in the correct direction. If not, the calculation should be stopped and started from the transition state again with a different sign for the integer specified after the .IRC keyword.

As the IRC is defined with respect to mass-weighted coordinates, care has to be taken in order to specify the correct isotopic substitution of the molecule. The specification of the isotopic constitution of a molecule is given in the MOLECULE.INP file, as described in Chapter 23.

Due to the small steps that must be used in a calculation of an IRC, such a calculation may require a large number of iterations, and it may thus be necessary to increase the maximum number of iterations that can be taken. This can be done by the keyword .MAX IT in the **DALTON input section. Default value for this parameter is 20 iterations.

All the information about the Intrinsic Reaction Coordinate will be collected in a file called DALTON.IRC. If a calculation stops because it has reached the maximum number of iterations, it may be restarted from that point, and the new information about the IRC will be added to the old DALTON.IRC file. This also implies that if a calculation is restarted from the beginning (because it went in the wrong direction) the DALTON.IRC file must be removed first. Thus it may often be useful to take a backup of the DALTON.IRC during the calculation of the IRC.

Finally, some comments on the interest of a calculation of IRCs. Whereas it will give results that mimic the behavior of what is considered to be a good description of the reaction pathway of a molecular reaction, it does not include any dynamical aspects of the reaction. There exists several models for approximating local regions of a molecular potential energy surface from the results of an IRC calculation, and from the potential energy surface valley, the dynamics of a chemical reaction may be mimicked. However, DALTON gives another, more direct, opportunity for studying molecular dynamics through dynamic walks as described in more detail in the next section.