As the IRC usually requires a normal coordinate, a force constant calculation normally has to be done first. If IRC is specified on its own, a normal coordinate is not used and the IRC calculation is performed on the supplied geometry.
A recommended sequence of operations to start an IRC calculation is as follows:
A DRC calculation is simpler, in that a force calculation is not a prerequisite; however, most calculations of interest normally involve use of an internal coordinate. For this reason IRC=n can be combined with DRC to give a calculation in which the initial motion (0.3kcal worth of kinetic energy) is supplied by the IRC, and all subsequent motion obeys conservation of energy. The DRC motion can be modified in three ways:
<filename>.res
can be edited to allow the user to modify the velocity vector or starting geometry. This file is formatted. Frequently, the DRC leads to a periodic, repeating orbit. One special type--the orbit in which the direction of motion is reversed so that the system retraces its own path--is sensed for and if detected the calculation is stopped after exactly one cycle. If the calculation is to be continued, the keyword GEO-OK will allow this check to be by-passed.
Sometimes the system will enter a stable state in which the geometry is always changing, but nothing new is occurring. One example would be a system which decomposed into fragments, and the fragments were moving apart. If all forces acting on the atoms become small, then the calculation will be stopped. If the calculation should be continued, then specify GNORM=0 LET.
Due to the potentially very large output files that the DRC can generate, extra keywords are provided to allow selected points to be printed. Two types of control are provided: one controls which points to print, the other controls what is printed.
Three keywords are provided to allow printing to be done whenever the system changes by a preset amount. These keywords are:
KeyWord |
Default |
User Specification |
0.05 Ångstroms |
X-PRIORITY=n.nn |
|
0.10 Femtoseconds |
T-PRIORITY=n.nn |
|
0.10 kcal/mol |
H-PRIORITY=n.nn |
For the IRC, the default is movement, i.e. X-PRIO, for the DRC, the default is T-PRIO.
By default, only the energies involved are printed (one line per point). To allow the geometry to be printed, LARGE is provided. Using LARGE a wide range of control is provided over what is printed.