[molpro-user] CASSCF energies using FROZEN

[Peterson, Kirk]

Dear Lorenzo,

no, I don't believe this is correct.  If you check your outputs carefully, you'll see that when you use frozen those orbitals are also always
doubly occupied in all CSFs, just like closed.  The orbitals are just not optimized.  So if you use frozen instead of closed, the
energy should always be higher in the frozen case (if both have converged to the global minimum).  One can see this with the very simple input:

geom={
C
O  1  2.1322}
hf
{multi;frozen,2}
{multi;closed,2}

Using  closed,2;frozen,2  will give the exact same energy as   closed,0;frozen,2 or just frozen,2

regards,

-Kirk

On Apr 13, 2013, at 8:18 AM, Lorenzo Lodi <l.lodi at ucl.ac.uk> wrote:

> I think I understand what was going on in my calculations, I was misinterpreting my inputs.
> 
> I'll explain my mistake, in case it might be useful for other people.
> A line such as
> CLOSED, 2; FROZEN,0;
> will create CSF where the two lowest orbitals are restricted to be doubly occupied, but the orbitals themselves are variationally optimized.
> On the other hand a line like
> CLOSED, 0; FROZEN,2;
> will impose no restriction on the CSF occupancy (hence producing a large active space than in the previous case) but will not optimize the two lowest orbitals.
> As the active space is larger it is quite possible that the total energy will be lower than in the previous case.
> What I really wanted to do was
> CLOSED, 2; FROZEN,0;
> where the active space is the same as in case 1 but the two lowest orbitals are not optimized. The total energy will then be higher than in case 1, as expected.
> 
> Lorenzo
> 
> 
> Il 12/04/2013 18:30, Lorenzo Lodi ha scritto:
>> Hello,
>> I have noticed that in some cases I get lower CASSCF energies when using
>> FROZEN orbitals than when specifying the same orbitals by CLOSED. I
>> would like to know if this is caused by convergence to a local minimum
>> of the CLOSED calculation or if this situation is somehow 'correct'.
>> Naively, I would expect that keeping orbitals FROZEN instead of CLOSED
>> would always increase the total energy. Am I wrong?
>> 
>> Lorenzo
> 
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