PC GAMESS/Firefly-related discussion club

Alex Granovsky
gran@classic.chem.msu.su

>>! Need extra states in CI as the second one is triplet              
>> $DET NCORE=7 NACT=2 NELS=2 NSTATE=4 GROUP=C1 ITERMX=200
>>PURES=.T. $END
>  I understand that PURES=.T. means that only singlet states are included in the
>state-averaging. Is this understanding correct?

Indeed, PURES=.T. means that only the states with spin
equal to that specified in input file will be included in
state-averaging procedure. Note that pures=.t. is the default
and strictly speaking it was not necessary to use this option.
Another point is that the default value of SZ is set according
to the MULT value in $contrl. However, this is the aldet CI code,
and thus it works with determinants rather than CSF.

This means it solves for NSTATE low-liyng roots regardless
of their multiplicity. The roots will be renumbered at the
subsequent stages for the purpose of state-averaging. Thus,
the second CI root is triplet, however, the second root in
averaging is indeed singlet (S1 state). This S1 state is
the third root of determinant CI.

> The sentence of 'the second one is
> triplet' makes me confused.
> Why the triplet state appears in this case?

Because this is determinant CI code. You can use GUGA
code, which works with CSF and will result in states
of only selected multiplicity. However, for large
active spaces ALDET is usually faster (well, the sample
itself is definitely not the large problem).

> Moreover,
> how did you determine the number of states  of four, that is,
> how did you determine
> NSTATE=4 before doing this run?
The answer is that for ALDET one usually needs some extra
roots to capture S1 (or SN). In this particular case of CAS (2,2)
it is evident that there exist only four states at all.

   
>! State-averaged MCSCF                                              
> $DET WSTATE(1)=1,1 $END  
>! istate=2 means we need properties and gradiends of second
> state (S1)  
> $mcscf CISTEP=aldet maxit=200 ISTATE=2 $end  
> The expression of 'second state (S1)' also makes me confused,
> because this seems to be inconsistent with the above sentence
> 'the second one is triplet'. I imagine that the triplet state is
> removed and thus the S1 state is 2nd lower state among all singlet
> states. Is this correct?

Exactly. With default pures settings, the CI roots are
renumbered so that the states of wrong spin are not counted.


> In opt.s1.sa.out  
> STATE 2  ENERGY= -113.6672881517  S=  1.00  SZ=  0.00  SPACE SYM=A
 
>ALPH|BETA| COEFFICIENT
>----|----|------------
> 01 | 10 |  -0.7071068
> 10 | 01 |   0.7071068
> 10 | 10 |   0.0000000
>
> I think that the above combinations of ALPHA and BETA
> should be of singlet states. But S=  1.00 is output. Why?  
> (I think that this means 2S+1=3)

This is the triplet state (exactly the same I meant while
mention that the second state is triplet). More precisely,
it's the triplet state having SZ=0.

Regards,
Alex Granovsky

[ This message was edited on Tue Nov 18 '08 at 5:00pm by the author ]