MOLPRO

20 THE CI PROGRAM 153
All states specified must be reasonably described by the internal configuration space. It is possible
to have different convergence thresholds for each state (see ACCU card). It is also possible
not to converge some lower roots which are included in the list nroot(i) (see REFSTATE card).
For examples, see REFSTATE card.
20.2.11 Defining reference state numbers
REFSTATE,nstatr,nrootr(1),nrootr(2),...,nrootr(nstatr);
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:
state,2;
will calculate two states with two reference states.
state,2;refstate,1,2;
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).
state,1,2;refstate,1,2;
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).
state,2;accu,1,1,1;
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.
20.2.12 Specifying correlation of orbital pairs
PAIR,iorb1.isy1,iorb2.isy2,np;
is a request to correlate a given orbital pair.
np=1:
np=-1:
np=0:
singlet pair
triplet pair
singlet and triplet pair (if possible)