I want to calculate the XANES spectrum of benzene at the Carbon K-edge using the CVS-ADC(2)-x method. As the six Carbon atoms are equivalent I want to use effective core potentials for five of the carbon atoms and then calculate transitions starting from only one of the C(1s) orbitals but I don’t know how to properly set my input parameters. Do I set CC_REST_OCC to 1 as there is only 1 Carbon atom left? And how can I tell the program that I want ECPs for only 5 (and not all) Carbon atoms?
Many thanks for helping!
What you want is something like ECP = MIXED (analogous to BASIS = MIXED that allows different basis sets for different instances of the same element). Unfortunately it looks like Q-Chem does not support that at present. ECP = GEN supports different types of ECPs (i.e., mixing and matching named pseudopotentials) but ECP = MIXED is not recognized, and that’s consistent with my recollection that this feature is not implemented.
A workaround for what you want to do would be to localize the MOs (e.g., Boys MOs will usually put the 1s on a single atom, even for symmetry-equivalent atoms), then use some kind of CVS-type (frozen orbital) approximation to exclude the C(1s) MOs that you don’t want to be active.
Calculations of the XAS spectrum of benzene and its cation is discussed here:
Regarding practical aspects of setting these calculations, you may find this webinar helpful:
We also encountered similar problems. We need to set different ECPs for the same element, ECP = MIXED(similar to BASIS = MIXED). Whether to consider adding this feature in the next release?
I have submitted a feature request, but I cannot make any promises about the timeline. (For developers, this is ticket #3014.)