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###

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.

** Next:** 20.2.12 Specifying correlation of
** Up:** 20.2 Specifying the wavefunction
** Previous:** 20.2.10 Defining state numbers
** Contents**
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molpro@molpro.net 2018-02-25