Posted by perake on November 21, 2001 at 08:37:29:

In Reply to: SDCI with several roots posted by Evgeniy on November 20, 2001 at 19:06:36:

Dear Evgeniy,

There are two problems here:

(1) When several roots are requested from the MRCI program, the start approximations

are taken from the reference CI. Thus, when you want several roots, the reference

space must be large enough so that it contains not just a single state. In fact, it

should be large enough so that the start approximations are reasonable ones, else

there unavoidable root flipping during iterations will make convergence problematic.

So the construction of the MRCI program makes it difficult to do a single-reference

SDCI if also excited states are needed. The calculated result would anyway be very

bad for the excited states, so is this really what you want to do? If you actually

need to do this calculation, please write me, and I will look into it.

(2) But you also want the roots for other symmetries. This is not possible with

the MRCI program, except of course that one may use a lower symmetry. But then the

entire calculation (SEWARD, SCF, etc) must be done in the lower symmetry. There is

presently no way of taking integrals and orbitals from a calculation using a certain

symmetry group and then compute wave functions that break this symmetry, nor can

RASSCF, MRCI or CASPT2 handle simultaneously wave functions of more than one irrep.

Best Regards,

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