A multireference coupled-cluster calculation using Mukherjee's ansatz (Mk-MRCC) is requested by the keyword
and is available at the Mk-MRCCSD, Mk-MRCCSD(T), and Mk-MRCCSDT level of theory. At the moment the use of a 2x2 CAS as reference space is permitted at the Mk-MRCCSD level of theory. For Mk-MRCCSD(T) and Mk-MRCCSDT, two closed-shell determinants can be used as reference.
The active orbitals in the CC treatment are either automatically defined (when using a TCSCF or ROHF reference,
core and active orbitals are then specified via the keywords CORE and ACTIVE or OCCUPATION, respectively) or have to be specified in a %mrccinput section (all other cases), where the irrep and the position within the irrep have to be given for each active orbital.
For Mk-MRCC calculation, the use of RHF, UHF, ROHF, and TCSCF orbitals is currently possible.
Analytic gradients for geometry optimizations and dipole moments are available at the Mk-MRCCSD and Mk-MRCCSDT levels of theory.
Harmonic vibrational frequencies can be evaluated at the Mk-MRCCSD level of theory via numerical differentiation of analytic gradients.