Optimization with frozen coordinates

[juanjo]

Hello:
I want to optimized two molecules together CF4 and SiNCS but freezing the intramolecular coordinates, that is, I want to optimize only the intermoleuclar coordinates. I think it is not possible to use Tmole in this case becuase with Tmole you can optimize only the total geometry.
But I cannot use define module to construct the input files because SiNCS is a linear molecule and with define it is not allowed to use dummy atoms. So, how can I do this kind of calculation?

[uwe]

Hi,

there is not need to define dummy atoms for SiNCS, since define uses internal coordinates, not Z-matrices.

By the way, dummy atoms are allowed in define, the are called 'q', but they are ignored when defining internal coordinates.

The probably simplest way to do what you want is to read in both molecules and let define generate automatically internal coordinates. If you generate non-redundant ones, just say iaut in the geometry menu and then *. Check with imet (after that the number of internal coordinates #k= must be the same as the number degrees of freedom #ideg=) if they are linearly dependent and look at them (disi). If you find a simple stretch from one fragment to the other one, fix all other internal coordinates by entering (i and then ic).

Example:

If you have, say, 20 internal coordinates, and coordinate number 11 is the one you would like to optimize while fixing all other ones, the command in the INTERNAL COORDINATE MENU would be

ic 1-10,12-20 f

After that, the first line in your current menu should look like that:

INTERNAL COORDINATE MENU  ( #ideg=20      #k=1    #f=19   #d=0   #i=0     )

while the d and i numbers might be different and can be ignored.

Another idea would be not to perform a geometry optimization, but do single-point jobs with different intermolecular distances.

Regards,

Uwe

[juanjo]

Thank you very much!!
It works well!!

[juanjo]

Hello:
In some cases when I try to generate automatically the internal coordinates with iaut, define gives the next message:

 isolated collinear chain detected!
 Bond Stretches will be assigned
 Attention: All nuclei must ly on the Z-Axis
 Symmetry must be at least C2v

and define says the number of internal coordinates is only three instead 21.

In this case, do I need to generate the internal coordinates manually?

Thank you!!