Restraints in optimisation

Questions
#1

Hi All,

I want to optimise a structure with some restraints. I would like to prevent atoms from moving in the ‘x’ cartesian direction.

Would the input be something like:
$opt
FIXED
1 X
2 X
etc…
ENDFIXED
$end

thank you!

#2

That is how you would do a restraint in X.

#3

So that will keep the X coordinates fixed?

#4

Yes. It will also force the optimizer to use Cartesian coordinates, which will likely take a lot of steps.

#5

I have input the restraints in the way specified, but when visualised they have moved so that the Cartesian axis are pointing in a different direction to the initial input, do you know why?

#6

Did you specify the following in your input?

SYMMETRY false
SYM_IGNORE true

#7

maybe also NO_REORIENT = TRUE, which forces Q-Chem to use the user’s lab-fixed frame, whereas by default the molecule is rotated into Standard Nuclear Orientation ( = principle axes of nuclear charge).