[molpro-user] Question on MRCI energies in multi-state calculations

[Lorenzo Lodi]

Dear Kirk,
thank you for your explanation, it's much clearer now.
As to the differences I noted when using different point-group symmetry 
I think they are due to the fact that the second reference state was not 
the same in C1/C2v.

Lorenzo

Il 30/06/2013 16:48, Peterson, Kirk ha scritto:
> Dear Lorenzo,
>
> this is from the  internal contraction.  By default the number of reference states for the internally contracted pairs is equal to the number of states requested.  So the more states you request, the more reference states used for the contracted pairs, and in general the lower the energy. You can modify this behavior with the refstate directive.
>
> best,
>
> -Kirk
>
>
> On Jun 30, 2013, at 3:58 PM, Lorenzo Lodi <l.lodi at ucl.ac.uk> wrote:
>
>> I noticed that when doing MRCI calculations (CASSCF reference) the energy of a certain state is different if that state is computed by itself or together with other, higher-lying states.
>> For example, in this mock calculation on CO:
>> geometry={C; O, C, 2.2}
>> {RHF}
>> {CASSCF}
>> {MRCI; STATE,1}
>>
>> I get
>> RHF=-112.743047
>> CASSCF=-112.879927
>> MRCI, STATE,1 = -113.047810
>> MRCI, STATE,2 = -113.047936
>> MRCI, STATE,3 = -113.048093
>>
>> The multi-state energies are consistently lower than the single-state one.
>> I also get a decrease in energy only for the multi-state MRCI calculations when doing the calculation without point-group symmetry (NOSYM keyword):
>> MRCI, STATE,2 (NOSYM)= -113.047896
>> MRCI, STATE,3 (NOSYM)= -113.048010
>>
>> Of course these energy difference are rather small (10-60 cm-1) but I would like to understand for what reason they exist at all.
>> Any hints?
>>
>> Lorenzo
>>
>>
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>