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Re^9: MRCI

Alex Granovsky

Dear Maksim,

most likely your problem is caused by a non-continuity of a
model space with respect to interatomic distance as described in
this thread:

Hope this helps.
Kind regards,
Alex Granovsky

On Sun Dec 7 '14 1:43am, Maksim Shundalau wrote
>Dear Alex,
>I thank you for your help. All these days Im trying to solving the problem with XMCQDPT2. I had some problems with CASSCF for ground and several excited states with 10 active orbitals. And now with 14 active orbitals I have smooth curves and negligible (about 0.2 cm-1) splittings for doubly degenerated Pi-terms.
>But with XMCQDPT2 I have the next problem: At some distance from the minimum (~ 4.09 A) curves lose smoothness and doubly degenerated Pi-terms are split into dozens cm-1. The distance at which this happens depends on the number of states averaged (in XMCQDPT2). If this number (N) is equal to 7 (number averaged singlets or triplets in CASSCF), then the breaks of the curve start near 4.3 A, with N=21 between 5.5 and 6 A, with N=28 between 6.5 and 7 A, with N=35 near 7 A, with N=90 between 7.5 and 8 A.
>Could you explain to me where I'm wrong?

>Best regards,
>On Tue Nov 11 '14 2:09pm, Alex Granovsky wrote
>>Dear Maksim,

>>>Then, in Molpro for MRCI level of theory I have the energies:
>>>-52.338309 (4.00 A), -52.338409 (4.10 A) and -52.338304 (4.20 A) and
>>>energy differences:
>>>-0.000100 [E(4.10)-E(4.00)] and +0.000105 [E(4.20)-E(4.10)].

>>>Thus, on passing from CASSCF to MRCI the energy decreases approximately 0.5 a.u. at each point, and minimum shifts from approximately 4.30 A to 4.00 A.

>>This is only possible if Molpro includes 8 lowest occupied orbitals
>>as double occupied inactive orbitals in the MRCI procedure.
>>This means that these orbitals are *not* frozen and that the single
>>and double excitations from these orbitals are allowed. This is
>>something I asked you about earlier. Note there are no inactive
>>orbitals in the SOCI procedure. With Firefly, the simplest way to
>>include inactive double occupied orbitals is to use XMCQDPT2.

>>Kind regards,

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