Editing G09 (section)

== Issues concerning the coordinate system ==

Some extra checks of the completeness of the coordinate system were added in Gaussian09. Thus in some cases you might get the error message "Error in internal coordinate system" where it did not appear in Gaussian03.

Typical cases where this error message appear, are for linear system. E.g. when three covalently bond atoms are arranged on a line. For organometallic systems a typical case is for carbonyl ligands coordinated to a metal centre, M-CO. But you can also find it in many other cases.

* If your system contains any linearly  arranged atoms and you get this error message, a solution might be to add linear bend coordinates to these atoms. In your modredundant section you write: '''L atom1 atom2 atom3 -1 A'''

* In the error message you will get the dihedral/angle affected, you can try to kill the problematic angle/dihedral ''' atom1 atom2 atom3 K'''

* '''Modify''' a bit the geometry, by changing a bit the angles/dihedrals or even just deleting from the third decimal of the xyz coordinates.

* To optimize in '''cartesian coordinates''' could be also a solution to this problem although your calculations are going to be slower and you are not going to use modredun (not possible to freeze and scan coordinated).

* Another solution is to use a Z-matrix and '''add ghost atoms''', that will allow you to define the system without angles and dihedrals close to 0 and 180.

* Revision C1 of Gaussian09 might have improved the way in which the internal coordinates are defined, as for some systems exactly the same input stops for RevA1 and runs without problems with '''RevC1'''. Be aware that RevC1 has issues with the modredundant section (see above)

* The Error does of course depend on the geometry, but also on the '''method/basis''', so the same geometry might give the error with, let's say, M06 and not with B3LYP. So you can optimize the geometry "a bit" with another method and then try again with the method you whish.