U1K

The sub key U1K determines which terms are included in the calculation of the U1 matrix (first order changes in MO coefficients).
Options (Default none):

Best
The best (recommended) options for each  relativistic option are included for this sub key.
All
Implies all the other options to this key.
MV
The mass-velocity term
Dar
The Darwin term
ZMAN
The Spin-Zeeman term.