[molpro-user] Computing higher roots in MRCI

[Gerald Knizia]

On Tuesday 14 April 2009 15:49, Gerald Knizia wrote:
> On Friday 10 April 2009 21:12, Jaffe, Richard L. (ARC-TSN) wrote:
>> [...] I can determine the first 6 roots, but I am trying to reduce
>> computational cost by obtaining accurate solution only for the 6th root.
>
> [...]  I'm not aware of any matrix diagonalization algorithm that can
> calculate a specific  eigenvector without also calculating all lower
> (or all higher) ones. 

I'm sorry for causing so much confusion by poorly formulating this (-^) 
statement. What I meant by ``specific'' was the property "the 6th 
eigenvector". While there are algorithms which can obtain an eigenpair 
closest to some input eigenvalue or eigenvector, one does need additional 
information about the rest of the spectrum to get a initial guess which 
actually converges to the nth root (and not some other one which happens to 
be near the input value).
However, in the CI case this information is available from the reference 
functions as  Alexander pointed out, so my "i guess this is impossible" 
comment is wrong.
-- 
Gerald Knizia