Dear Turbomole Users,
In the webpage
http://www.uni-koeln.de/themen/chemie/software/turbomole/DOK/html/node107.html (Version 5.6) there is a suggestion that one can constrain the spin of a UHF wavefunction calculation by writing in the control file:
"$uhf {spin constraint={real}}"
if I understood correctly, which I’m not sure I have, {real} should be the value one wishes the wavefunction to give for the S
2 operator.
If that is right, I am not sure I understood the format It requires for one to define the {real} variable.
I wished to optimise a doublet (s=1/2) state, so that S
2 = s(s+1) = 0.75.
I tried "$uhf 0.75", "$uhf spin constraint=0.75", and a few other combinations.
On all occasions, I got the warning:
"*** WARNING: Could not read properly <logical> value for $uhf ***"
On the web, I’ve seen output files that print the following statement:
"UHF modus switched on ! SPIN CONSTRAINT is : tau= .00000"
Which suggests this can be done. On the other hand, the pdf version of the manual does not mention this option, which made me wonder whether the option exists.
I have run calculations where I start with a wavefunction that gives S
2 = 0.75.
but by the end of the RIDFT-MARIJ calculation I often get values above 2, presumably due to spin contamination.
Is there a way to constrain the spin during the optimization?
Thank you for your time!