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)].