20.2.11 Defining reference state numbers


nstatr is the number of reference states for generating contracted pairs. This may be larger or smaller than nstate. If this card is not present, nstatr=nstate and nrootr(i)=nroot(i). Roots for which no reference states are specified but which are specified on the STATE card (or included by default if the nroot(i) are not specified explicitly on the STATE card) will not be converged, since the result will be bad anyway. However, it is often useful to include these states in the list nroot(i), since it helps to avoid root flipping problems. Examples:

will calculate two states with two reference states.
will optimize second state with one reference state. One external expansion vector will be generated for the ground state in order to avoid root flipping. The results printed for state 1 are bad and should not be used (unless the pair space is complete, which might happen in very small calculations).
As the second example, but no external expansion vectors will be generated for the ground state. This should give exactly the same energy for state 2 as before if there is no root flipping (which, however, frequently occurs).
Will calculate second state with two reference states. The ground state will not be converged (only one iteration is done for state 1) This should give exactly the same energy for state 2 as the first example.
molpro@molpro.net 2018-09-21