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21.3 Multi-State CASPT2

Multi-state CASPT2 is implemented as described by Finley et al. CPL **288**, 299 (1998).
Currently this can only be used with the `RS2` program (i.e., not with `RS2C`).
There are two different modes in which MS-CASPT2 calculations can be performed:

(i) Each of the states to be mixed is computed independently,
and finally all states are mixed. In the following, such
calculations will be denoted SS-SR-CASPT2 (single-state, single reference CASPT2).
There is one contracted reference state for each CASPT2 calculation that is specific
for the state under consideration. This is the cheapest method, but
there are no gradients available in this case. It is the users responsibility
to make sure that no state is computed twice.

(ii) All *nstates* states are treated together, with *nstates* contracted reference
states. This is more expensive, but should give a more balanced description
since the different reference states can mix in the CASPT2. It is required
that *nstates* equals the number of states specified on the `STATE` directive.
For this case, denoted ”MS-MR-CASPT2” (multi-state multi reference CASPT2), analytical energy gradients are available,
see section 21.7. It is recommended to use the "extended" (XMS) multi-state CASPT2 option,
which guarantees invariance of the theory with respect to unitary rotations of the reference functions.
The method yields improved potentials in the vicinity of avoided crossings and conical intersections [see
T. Shiozaki and H.-J. Werner, J. Chem. Phys. **133**, 141103 (2010) and
T. Shiozaki, C. Woywod and H.-J. Werner, Phys. Chem. Chem. Phys. **15**, 262 (2013)].

**Subsections**
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